AbstractA preliminary study of the pharmacokinetics of methylmercury was performed in sheep after a single intravenous dose of203Hg‐labeled methylmercury. Blood samples were taken periodically, and plasma and whole blood203Hg were determined. Blood and plasma203Hg concentrations exhibited a biphasic exponential decline. The half‐time for the major component was 14.6 days for plasma203Hg and 14.1 days for whole blood. There was a uniform distribution of203Hg between red cells and plasma throughout the study. A red cell to plasma concentration ratio of 14.9:1 was obtained, with approximately 88% of the203Hg in blood associated with the red cell fraction. Both the feces and the wool represented major routes of excretion. Approximately 18% of the dose was present in the wool taken on day 8. This is compared with a total excretion in the feces of 11.5% of the dose over the same time course. The total of 29.5% compares favorably with a reduction of 32.5% in the terminal component of the whole blood curve. Sequential analysis of wool from the root end revealed increasing concentrations of203Hg to a peak. Using the first appearance of the203Hg label in the wool as a marker to determine the wool growth rate, the203Hg in wool shewed an exponential decline with a half‐time similar to that seen for whole blood and plasma. The wool to blood ratio was estimated to be approximately 120:1. Tissue analysis at sacrifice revealed a rather uniform distribution of203Hg label in the brain. Somewhat lower levels were observed in the spinal cord and ganglia. Kidney had the highest203Hg concentration, and edible muscle had approximately six times the concentration measured in blood. The concentration of203Hg was three times higher in bile than in plasma, consistent with the substantial fecal excretion observed. The sheep may be a useful animal model for studying regional deposition of methylmercury in the CNS and mechanisms of methylmercury deposition into