Biotransformation of methyl mercuric salts in the mouse studied by specific determination of inorganic mercury

The immunosuppressive effect of methylmercury does not preclude development of autoimmunity in genetically susceptible mice

Methylmercury (MeHg) is a common environmental pollutant due to both natural and anthropogenic sources. Although the central nervous system (CNS) is considered the critical organ for the toxic effect of MeHg, it has recently been suggested that the immune system might be at least as sensitive as the CNS. We have examined the effects of MeHg on the immune system in genetically metal-susceptible mice. Subcutaneous (sc) injections of 2 mg MeHg/kg body weight (bw) every third day (internal dose ca. 540 microg Hg/kg bw/day) to A.SW mice of the H-2(s) haplotype, caused during the first week a 47 and 9% reduction of B- and T-cells, respectively, which indicates immunosuppression. Subsequently, an autoimmune syndrome developed which shared certain features with the syndrome induced by inorganic mercury in H-2(s) mice, including antibodies targeting the 34 kDa nucleolar protein fibrillarin, increased expression of IL-4 mRNA, increase of Th2-type of immunoglobulins (IgE and IgG1), and increased MHC class II expression on B-cells. However, the response using MeHg was attenuated compared with even lower doses of Hg in the form of inorganic mercury, and specifically lacked the increased expression of IL-2 and IFN-gamma mRNA, the polyclonal B-cell activation (PBA), and the systemic immune-complex (IC) deposits which are induced by inorganic mercury. Increasing the dose of MeHg increased the titre of anti-nucleolar antibodies and shortened the induction time, but did not lead to stronger immunostimulation or systemic IC-deposits. The kidney and liver selectively accumulated MeHg, while the blood, spleen and lymph nodes showed lower levels of MeHg. The accumulation of MeHg and Hg(2+) increased throughout the 30-day period. The fraction of Hg(2+) in the kidney varied between 4 and 22%, and the lymph nodes showed a maximum of 30% Hg(2+). We conclude first that MeHg has quantitatively different effect on the immune system compared with inorganic mercury, and secondly that an initial immunosuppression induced by a xenobiotic does not preclude subsequent immunostimulation and autoimmunity.

Methyl mercury-induced autoimmunity in mice

Female SJL/N, A.SW, B10.S (H-2s), BALB/C, DBA/2 (H-2d), A.TL and B10. TL (H-2t1) mice were treated with sc injections of 1.0 mg CH3HgCl/kg body weight every third day for 4 weeks. Controls were given sterile, isotonic NaCl. CH3HgCl (MeHg) induced in SJL, A.SW and B10.S mice antinucleolar antibodies (ANoA) targeting the nucleolar 34-kDa protein fibrillarin. The susceptibility to develop ANoA in response to MeHg was linked to the mouse major histocompatibility complex (H-2), since H-2s but not H-2t1 mice sharing background (non-H-2) genes developed ANoA. However, the background genes decided the strength of the ANoA response in the susceptible H-2s mice, and the ANoA titer was in the order: A.SW > SJL > B10.S. Although MeHg as well as inorganic mercury induced ANoA, the two forms of mercury differed both quantitatively and qualitatively in their effect on the immune system. MeHg induced in H-2s mice a weaker general (polyclonal) and specific (ANoA) B-cell response than HgCl2, probably due to weaker activation of Th2 cells with lower IL-4 production, as indicated by the minimal increase in serum IgE. The A. TL strain with a susceptible genetic background, but a H-2 haplotype resistant to HgCl2, responded to MeHg with a modest polyclonal B-cell response dominated by Th1-associated Ig isotypes. H-2s mice treated with MeHg showed in contrast to HgCl2-treated mice no systemic immune-complex (IC) deposits, which may be due to the weaker immune activation after MeHg treatment. The increase in serum IgE concentration and ANoA titer 2-6 weeks after stopping treatment with MeHg is identical to reactions during the first 2-3 weeks of HgCl2 treatment. Therefore, demethylation of MeHg probably increased the concentration of inorganic mercury in the body sufficiently to reactivate the immune system. This reactivation indicated that genetically susceptible mice are not resistant to challenge with mercury, making them distinctly different from rats.

Organic mercury: an environmental threat to the health of dietary-exposed societies?

As a natural element, mercury is ubiquitous in the environment. The largest amount of mercury, amounting to approximately 100,000 tons per year, originates from the degassing of the earth's crust. To this amount, such anthropogenic activities as combustion of fossil fuels and releases from industrial activities add approximately 20,000 tons of mercury every year. The emitted mercury, both natural and anthropogenic, is in an inorganic form, predominantly as the metallic vapor (Hgzero). In aquatic environments, however, inorganic mercury is microbiologically transformed into the lipophilic organic compound, methylmercury. The transformation from the hydrophilic to the lipophilic state makes mercury more prone to biomagnification in aquatic food chains. Consequently, populations with a traditionally high dietary intake of food originating from either fresh-water or marine environments have the highest exposure to methylmercury. Because of their traditional pursuit of marine mammals, the Inuits belong to the highest dietary exposure group /1/. This situation is particularly true for the Polar Eskimos in North West Greenland. This population has the most traditional lifestyle among the Inuits and hunts predatory species of whales, such as beluga and narwhal, a combination that results in a high level of exposure to methylmercury. Polar Eskimos in North West Greenland, living in areas with no 'accidental' mercury pollution, but with a high dietary access to methylmercury thus exemplify a population group with a current potential environmental health problem.

Metabolism of mercury in hamster pups administered a single dose of 203Hg-labeled methyl mercury

Golden Syrian hamster pups were administered a single subcutaneous dose of 203Hg-labeled methyl mercury (MeHg), 0.4 nmol/g body weight, seven days after birth, and were sacrificed 2, 7, 14, 21 or 28 days later. The excretion of 203Hg followed a biphasic elimination pattern with an average half-time of 8.7 days for the rapid component. The slow component had a much longer half-time and probably reflects binding of 203Hg to growing hair. The concentration of 203Hg in the liver, kidneys and brain two days after administration was 0.44, 0.38 and 0.19 nmol/g, respectively. The retention of 203Hg was higher in the kidney than in the liver and the brain. The content of inorganic 203Hg in the liver and kidneys increased the first weeks after administration, demonstrating that hamsters are able to demethylate MeHg before two weeks of age.

Altered intrarenal accumulation of mercury in uninephrectomized rats treated with methylmercury chloride

We tested the hypothesis that the intrarenal accumulation of mercury in rats treated with methylmercury is altered significantly as a result of unilateral nephrectomy and compensatory renal growth. Renal accumulation of mercury was evaluated by radioisotopic techniques in both uninephrectomized (NPX) and sham-operated (SO) rats 1, 2, and 7 days after the animals received a nonnephrotoxic intravenous dose of methylmercury chloride (5 mg/kg Hg). At all times studied after the injection of the dose of methylmercury, the renal accumulation of mercury (on a per gram kidney basis) was significantly greater in the NPX rats than that in the SO rats. The increased accumulation was due to a specific increase in the accumulation of mercury in the outer stripe of the outer medulla. Renal cortical accumulation of mercury was similar in both the NPX and SO rats. The percentage of the administered dose of mercury that was present in the total renal mass of the NPX and SO rats ranged between 5 and 15, depending on the day that the renal accumulation was studied. Approximately 40-50% of the total renal burden of mercury in both the NPX and SO rats was in the inorganic form. However, only less than 1% of the mercury in blood was in the inorganic form at the three times accumulation was studied. Very little mercury was excreted in the urine by either the NPX or SO rats. Only about 2 to 3% of the administered dose of mercury was excreted in the urine in 7 days. By contrast, the cumulative fecal excretion of mercury over 7 days was substantial in the NPX and SO rats, and significantly more mercury was excreted in the feces by the NPX rats (about 19% of the dose) than by that in the SO rats (about 16% of the dose). In conclusion, our findings indicate that unilateral nephrectomy and compensatory renal growth cause a significant increase in the accumulation of mercury in the renal outer stripe of the outer medulla in rats exposed to methylmercury. In addition, the findings indicate that the fecal excretion of mercury is also significantly increased.

Routes for renal transport of methylmercury in mice

To clarify the routes for renal methylmercury uptake, the effects of ureter ligation and pretreatment of probenecid, an organic anion transport inhibitor, or acivicin, a gamma-glutamyltranspeptidase (gamma-GTP) inhibitor, on renal methylmercury content were investigated in mice. For 120 min after CH3HgCl (5 mumol/kg, i.v.) injection, renal methylmercury content in bilateral ureter-ligated mice was approximately 50% lower than that of sham-operate mice. The glomerular filtration rate was reduced to about 15% of the control by ureter ligation. These results suggest an important role of glomerular filtration in the renal methylmercury uptake. Pretreatment with probenecid (0.5 or 1.0 mmol/kg, i.p.) reduced the renal methylmercury accumulation 30 min after CH3HgCl injection in a dose-dependent manner in both ureter-ligated and sham-operated mice. Urinary methylmercury excretion was not affected by probenecid pretreatment. Renal methylmercury content of ureter-ligated mice was not changed by pretreatment with acivicin (0.5 or 1.0 mmol/kg. i.p.), which was previously reported to decrease the renal methylmercury content in mice. Coadministration of GSH (10 mumol/kg, i.v.) with CH3HgCl increased the renal methylmercury uptake determined 5 min after injection in ureter-ligated mice. These results suggest that at least two transport systems play major roles in renal methylmercury uptake: one is a route from the glomeruli through the brush border membrane which is dependent on the action of gamma-GTP, and the other route is the one using an organic anion transport system through the basolateral membrane.

Influence of dietary protein levels on the fate of methylmercury and glutathione metabolism in mice

We investigated the influence of dietary protein levels on the fate of methylmercury (MeHg), the tissue glutathione (GSH) levels and the efflux rates of GSH in C57BL/6N male mice. One group of mice was fed a 7.5% protein diet (low protein diet, LPD) and the other was fed a 24.8% protein diet (normal protein diet, NPD). The cumulative amount of Hg in urine in LPD-fed mice was approximately 3.7-times lower than in NPD group during the 7 days after oral administration of MeHg (20 mumol/kg), although the fecal Hg levels were identical in the two groups. Hg concentration in kidney, liver and blood decreased time-dependently for 7 days after the administration in both groups of mice, whereas the brain levels continued to increase during this period. Tissue Hg levels in the LPD group were significantly higher than in the NPD group except for the liver. Although the hepatic GSH level in LPD-fed mice was significantly lower than in NPD-fed mice, the levels in the kidney, brain, blood and plasma were not different between the two groups. The efflux rate (mumol/g body weight per day) of hepatic GSH in LPD-fed mice was significantly lower than in the NPD group, whereas the efflux rates of renal GSH were identical in both groups. When MeHg (20 mumol/kg)-pretreated mice were injected with acivicin, a specific inhibitor of gamma-glutamyltranspeptidase, the urinary Hg levels increased by 60- and 36-fold in groups fed LPD and NPD, respectively. As a result, the difference in urinary Hg levels between the two groups disappeared with acivicin treatment. This result indicated that LPD feeding might decrease urinary Hg excretion by increasing the retention of MeHg metabolite(s) in renal cells. Thus, our present study suggested that the dietary protein status, which could modulate the metabolism of thiol compounds, played an important role in determining the fate of MeHg.

Possible role of hepatic glutathione in transport of methylmercury into mouse kidney

The mechanism of the renal uptake of methylmercury was studied in mice. Preadministration of 1,2-dichloro-4-nitrobenzene (DCNB), which is a reagent that depletes hepatic glutathione (GSH) without affecting the renal GSH level, 30 min before injection of methylmercury significantly decreased the renal accumulation of mercury. The renal accumulation of mercury in mice receiving methylmercury-GSH intravenously was significantly higher than that in mice receiving methylmercuric chloride. These results suggest the possibility that hepatic GSH, as a source of extracellular GSH, plays an important role in the renal accumulation of methylmercury. No significant difference in renal mercury accumulation between bile duct-cannulated mice and normal mice was observed, indicating that the enterohepatic circulation of methylmercury is not an important factor in the renal accumulation of methylmercury in mice. Pretreatment of mice with acivicin, a potent inhibitor of gamma-glutamyl transpeptidase (gamma-GTP), significantly depressed the renal uptake of methylmercury and increased the urinary excretion of GSH and methylmercury. In in vitro reactions, methylmercury-GSH was degraded into methylmercury-cysteinylglycine by gamma-GTP, and this product was then converted to methylmercury-cysteine by dipeptidase. These results suggest that methylmercury is transported into the kidney as a complex with GSH, and then incorporated into the renal cells after degradation of the GSH moiety by gamma-GTP and dipeptidase, although the methylmercury bound to extracellular GSH can be reversibly transferred to plasma proteins in the bloodstream.

Enhanced and inhibited biotransformation of methyl mercury in the rat spleen

Biotransformation of methyl mercury in rats was studied by enhancing or inhibiting its biotransformation with various procedures. A new sensitive method developed to determine specifically inorganic mercury in the presence of organic mercury was used. Biotransformation was enhanced by treating the rat with phenylhydrazine. The increase of inorganic mercury was highest (four to five times) and rapid in the spleen. Inhibited biotransformation of methyl mercury was observed in splenectomized rats. The inorganic portion of total mercury in the macrophage-rich fraction of spleen cells was clearly higher than that in unfractionated spleen cells. The biotransformation of methyl mercury was inhibited by treating the rat with carrageenan, a well-known substance blocking macrophage function. These results suggest that the spleen is an important site for the formation of inorganic mercury, and that the macrophage participates in this biotransformation.

Involvement of glutathione in the enhanced renal excretion of methyl mercury in CFW Swiss mice

The present studies attempted to identify the mechanism for the elevated urinary excretion rate for methyl mercury (MM) previously reported in CFW Swiss mice. Strain comparisons of factors which could conceivably influence renal excretion of MM were made. The biotransformation of MM to the inorganic form did not appear to play a significant role. No significant strain differences were observed in the distribution of MM between plasma and red cells under in vivo or in vitro conditions. The percentage of total plasma MM present in the low-molecular-weight fraction did not differ statistically between the CFW and CBA/J strains. Strain comparisons of total reduced nonprotein thiol concentrations in liver, kidneys, whole blood, and plasma revealed no significant strain differences. A significant strain difference in plasma oxidized glutathione (GSSG) concentrations was observed. However, plasma concentrations of reduced glutathione (GSH), the form of glutathione (GS) which interacts with MM, did not significantly vary between the strains. The rate of total glutathione (TGS) excretion in urine was approximately 2-fold higher in CFW mice than in CBA/J mice. The significantly higher urinary GS excretion in CFW mice was accompanied by a 1.6-fold lower urinary gamma-glutamyltranspeptidase (gamma-GTP) activity in this strain.

Species difference in biliary excretion of methylmercury

Species difference in the biliary excretion of methylmercury was studied in male rats, mice, rabbits and guinea pigs. The rates of mercury excretion (% dose/2 hr) into the bile of the rats, mice, rabbits and guinea pigs during the 2 hr from 2 to 4 hr after the administration of methylmercury were 0.61, 0.091, 0.036 and 0.019, respectively. These results suggest that biliary excretion and enterohepatic circulation of methylmercury in the latter three species may not influence the fate of this compound as significantly as in rats. Most of the methylmercury excreted into the bile of rats was bound to glutathione (GSH). In the mouse bile, 40% of the methylmercury was bound to GSH and the rest was found in a fraction eluted at the void volume of the Sephadex G-15 column. However, in the case of the rabbits and guinea pigs, methylmercury-GSH was scarcely detectable in the bile and almost all of the methylmercury was eluted at the void volume of the column.

Cellular and subcellular demonstration of mercury in situ by modified sulfide-silver technique and photoemulsion histochemistry

Young adult C57BL/6J mice were injected with 4.0 mg/kg body wt of methylmercuric chloride (MMC) for 3 consecutive days for a total of 12.0 mg/kg. Control animals received physiological saline in place of MMC. One week following the last injection, the animals were sacrificed. Representative tissue blocks and sections from the brain, kidney, and liver were subjected to a modified sulfide-silver technique (SST) and the photoemulsion histochemical method. The results show that both of these techniques demonstrate consistent and distinct localization of mercury (Hg) gains in cells and subcellular organelles. These methods are based on the principle that Hg compounds react strongly with sulfhydryl groups in tissues and cells to form Hg-sulfides and also on the affinity of Hg and silver to form an amalgam when placed in a physical developer or photographic emulsion. Thus Hg in cells is demonstrable without prior treatment of sulfide solution. The methods are simple and reliable when used with proper controls. Specific localization of Hg in cells in situ without the use of radioactive material and without disruption of anatomical relationships provided by these methods offers distinct advantages over other methods of Hg determination. Thus it would be possible to conduct a retrospective or prospective study of human autopsy materials and also would allow direct correlation of Hg deposition with pathological changes in cells and subcellular organelles.

Co-enzyme effects of inorganic mercury in the liver of a freshwater fish Channa punctatus

Mercury is known to modify enzyme activity through oxidation of thiol groups and respective reverse reactions in vitro and in vivo. However, variations in the activity of carbohydrates, and the significance of this variation after mercury poisoning in different species, has not been established. In the present report, the effects of inorganic mercury on selected hepatic enzymes was studied in the freshwater fish Channa punctatus. Quantitative data clearly showed a dose-response relationship between the amount of mercury retained in the liver and inhibition of enzymes (i.e. alkaline phosphatase, glucose-6-phosphatase, amylase, maltase, lactase, lipase and dehydrogenases). Mechanisms and significance of their modification have also been discussed.

Methyl mercury decomposition in mice treated with antibiotics

The role of intestinal flora in the decomposition and faecal excretion of methyl mercury was studied in mice treated with antibiotics. The antibiotics, neomycin sulfate and chloramphenicol, were given to mice in drinking water for six days before intraperitoneal administration of methyl mercuric chloride (MMC), and intestinal microorganisms were thereby reduced. Inorganic and organic mercury were determined separately for faeces, intestinal contents and organs. On the fourth day after the mercury administration, the percentage ratios of inorganic mercury to total mercury in the contents of the caecum and large intestine were less in the mice treated with antibiotics, at 37% and 39%, respectively, than in the control mice (66% and 65%, respectively). Administration of the antibiotics reduced the excretion of inorganic mercury in the faeces to 26% of that of control mice and also reduced the excretion of total mercury to 60%. Reduction of intestinal microorganisms by the antibiotics was assumed to have caused the reduced decomposition of methyl mercury in the caecal contents and the reduced excretion of total mercury in the faeces.

Toxicokinetics and methyl mercury in pigs

Toxicokinetics of methyl mercury were studied in pigs after intravenous (i.v.) administration of the compound. The distribution of methyl mercury was slow taking 3-4 days to be completed. Blood elimination half-life was found to be 25 days. The apparent volume of distribution was 9.8 l/kg indicating pronounced tissue accumulation of methyl mercury. Highest mercury levels were found in kidney and liver, with lower contents in muscle and brain and very little in adipose tissue. The results indicate that from organs like liver and kidney methyl mercury is eliminated much more slowly than from the blood. Over a period of 15 days 16% of the dose administered was excreted with faeces and 0.9% in the urine.

Distribution and biotransformation of methyl mercuric chloride in different tissues of mice

The distribution of 203Hg radioactivity has been studied in various organs of adult male and female mice from one hour to 21 days after treating with 203 Hg-labeled methyl mercuric chloride (MMC). The amount of methyl mercury (MeHg) and inorganic mercury (Hg) has also been determined by injecting single doses of non-radioactive MMC, and subsequently measuring total, organic and inorganic Hg content by atomic absorption technique. In addition, photoemulsion histochemical method (PEHM) was used to demonstrate localization of Hg grains in various cellular compartments of organs and tissues. The highest levels of radioactivity were attained at 7 hours post-treatment in all organs except for brain and testis. The testis showed the highest radioactivity at one day and the brain at two days post-treatment. MeHg persisted in brain over a longer period though the level was not as high. The content of MeHg and inorganic Hg was maximum in kidneys as compared to other organs. The brain and the reproductive organs contained the least amount of inorganic Hg. By PEHM, Hg grains were most prominently observed in the sinusoids, Kupfer cells, hepatic cells and bile duct epithelium of liver; in the lumen of blood vessels, convoluted and collecting tubules of kidneys; and in the gastrointestinal epithelium. The pattern of uptake and distribution of MeHg correlated well with the morphological demonstration of Hg grains in tissue sections.

Treatment of methyl mercury poisoning in mice with 2,3-dimercaptosuccinic acid and other complexing thiols

Treatment with 2,3-dimercaptosuccinic acid was more effective than N-acetyl-DL-penicillamine and monomercaptosuccinic acid in mobilizing mercury from mice after the injection of methyl mercuric chloride. Dimercaptosuccinic acid treatment started 4 days after the mercury injection and given for 8 days at a dose of 1 mmol SH/kg per day removed more than 2/3 of the mercury in the brain, while acetylpenicillamine and mercaptosuccinate correspondingly removed less than 1/2 of the brain deposits. Neither treatment with 2,3-dimercaptorropano-1-sulphonate nor with a new thiolated resin, mercaptostarch, mobilized significant amounts of mercury from the brain. Since the toxicity of dimercaptosuccinate seems to be almost as low as that of D-penicillamine this dithiol may provide a potentially useful agent in clinical poisoning due to methyl mercury.

Influence of protein or cystein deficiency on hepatic subcellular distribution of methyl mercury in two rat strains

The influence of protein deprivation and cystein deficiency on the distribution of methyl mercury between 4 subcellular fractions of liver was studied in 2 rat strains (Wistar, strain R and Sprague-Dawley). Kept on a standard diet, the 2 strains showed a similar distribution pattern, with the highest mercury level found in the cytosol, followed by the mitochondrial, microsomal and nuclei fractions. The protein free diet caused on increase in the total amount of bound mercury in both strains, the greatest increase, being found in livers from strain R rats. The cystein deficient diet, on the other hand, gave rise to diverging results. Whereas the level of mercury bound to the subcellular fractions was increased in livers from strain R rats, it was markedly reduced in livers from Sprague-Dawley rats.

Chronic toxicity of methylmercury in the adult cat. Interim report

Doses of 3, 8.4, 20, 46, 74 or 176 mug Hg/kg/day were fed to groups of 8--10 adult cats, either as methylmercuric chloride or as methylmercury-contaminated fish, 7 days/week for up to 2 years. Food consumption, body weight change, blood mercury levels, haematology, urine analysis, serum blood urea nitrogen (BUN) levels and neurological status were assessed regularly in all animals. Clinical signs of methylmercury toxicity -- consisting of ataxia, loss of balance and motor incorrdination -- occured in groups receiving 176 mug Hg/kg/day after 14 weeks of treatment. Pathological findings were confined to the nervous system and consisted of loss of nerve cells with replacement by reactive and fibrillary gloisis. Terminal blood and brain mercury levels were approx. 10 ppm. There were no differences in the time required to develop clinical signs of methylmercury toxicity, tissue mercury levels or pathology between the groups of cats receiving methylmercury as methylmercuric chloride or as methylmercury-contaminated fish, at either dose level. Blood mercury levels in the remaining doses groups appeared to plateau after 40 weeks of treatment. Groups receiving 46 mug Hg/kg/day began to show some neurological impairment after 60 weeks of treatment which did not progress in subsequent weeks. No treatment-related effects were present in groups receiving 20, 8.4 or 3 mug Hg/kg/day after 2 years.

Metabolism and toxicity of cadmium, mercury, and lead in animals: a review

Cadmium, mercury, and lead are toxic to humans and animals. Although cadmium and inorganic mercury toxicities occur in humans, they have not been observed in domestic livestock under practical conditions. In contrast, cattle, especially young calves, are extremely susceptible to lead toxicity. Apparently, cattle are more tolerant of cadmium than are other animal species. Due partially to higher absorption and longer retention times in the body, the alkyl mercuries, especially methyl mercury, are more toxic than inorganic mercury compounds. Inorganic forms of cadmium, mercury, and lead are poorly absorbed from the intestine. However, due to lack of effective homeostasis, after absorption retention time is long. Injected cadmium, mercury, and lead are metabolized differently from that naturally absorbed. Most cadmium and mercury are in kidney and liver (50 and 23% of total body in goats); but highest total load of methyl mercury is in muscle (72% in cows). With low to moderate body burden, most lead is retained in the skeleton. However, beyond a certain point, the kidney accumulates large quantities. Only minute amounts of cadmium and mercury are secreted into milk, but milk is only moderately well protected from dietary lead. Likewise, little cadmium and inorganic mercury pass the placental barrier whereas lead and methyl mercury pass more readily.

Dose-dependence of methylmercury metabolism. A study of distribution: biotransformation and excretion in the squirrel monkey

The distribution and excretion of different body burdens of methylmercury (MeHg) have been investigated in the squirrel monkey. In monkeys given weekly 0.8 mg/kg doses, orally, of 203-MeHg, a linear correlation was observed between the concentrations of radioactive Hg in the blood and brain to as much as a blood concentration of 1 mug/gm. Above this level, the ratio of concentration in the brain and blood was increased. The total Hg concentration in bile collected from the bile duct was 10% to 30% of that in blood, while the concentration in bile from the gallbladder approached that in the blood. The total Hg concentration in feces was always more than ten times that in urine. Biotransformation of MeHg to inorganic mercury has been demonstrated; in the liver about 20% of the total mercury was inorganic, in the kidney 50%, and in the bile 30% to 85%. In the brain less than 5% of the total mercury was inorganic. After a single 0.8 mg/kg dose, orally, of 203-MeHg, the halftime for total Hg in blood was 49 plus or minus 2.8 days, and in the whole body 134 plus or minus 2.7 days. During the first four days after dosing, the decrease in blood concentration was more rapid than that occurring later, due to a redistribution within tissue compartments. A differential distribution of MeHg within the brain has been demonstrated in animals that showed clinical signs of intoxication.