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With the development of a rapid assay for arsenite methyltransferase (Zakharyan et al., 1995), the specific activity of this critical enzyme for arsenite biotransformation was determined by incubating liver, testis, kidney, or lung cytosol of male B6C3F1 mice with sodium arsenite and S-[methyl-3H]adenosyl-L-methionine and measuring the formation of [methyl-3H]monomethylarsonate. The mean arsenite methyltransferase specific activities (U/mg +/- SEM) measured in these organs were liver, 0.40 +/- 0.06; testis, 1.45 +/- 0.08; kidney, 0.70 +/- 0.06; and lung, 0.22 +/- 0.01. Heretofore, the enzymatic methylation of arsenite has been regarded primarily as a hepatic function. The arsenite methyltransferase specific activity of the testis was 3.6 times greater than that of the liver (p0.01) and the specific activity of the kidney was 1.8 times greater than that of the liver (p0.05). Additionally, when mice were given arsenate in drinking water for 32 or 91 days at concentrations of 25 or 2500 micrograms As/L, the arsenite methyltransferase activities of liver, testis, kidney, and lung cytosol were not significantly increased in animals receiving either dose of arsenic for either 32 or 91 days compared to controls. No evidence for the induction of arsenite methyltransferase was found under these experimental conditions.