The effects of meso 2, 3-dimercaptosuccinic acid (DMSA), sodium 2, 3 dimercaptopropane 1-sulfonate (DMPS) and S-adenosyl L-methionine (SAM) on the enzymatic activities of mice were studied. The mice were given intraperitoneal (i. p. ) injections of these chelating agents (1 mmol/kg) and 3 h later the activity of 8-aminolevulinic acid dehydratase (ALAD) in the blood, and aspartate aminotransferase (AST), alanine aminotransferase (ALT), γ-glutamyltranspeptidase (γ-GT), alkaline phosphatase (ALP) in the liver and kidney were determined. The activity of blood ALAD was significantly increased by the administration of DMSA and SAM while DMPS had only a moderate effect. The activities of other hepatic enzymes changed little when the mice were treated with these chelating agents, except for a significant reduction in hepatic ALP activlty following DMPS administration. Arsenic (Ⅲ) administration markedly increased the activities of ALT and ALP in the liver and kidneys. The changes in the enzymatic activities by treatment with arsenic were prevented by injection of DMSA,DMPS and SAM, DMSA being the most effective. These results indicate that DMSA, DMPS and SAM were not toxic to the liver or kidneys of mice and that treatment with DMSA is more effective than DMPS or SAM in protecting mice from acute hepatic or renal toxicity caused by arsenic. The effects of meso 2, 3-dimercaptosuccinic acid (DMSA), sodium 2, 3 dimercaptopropane 1-sulfonate (DMPS) and S-adenosyl L- methionine (SAM) on the enzymatic activities of mice were studied. The mice were given intraperitoneal ip) injections of these chelating agents (1 mmol / kg) and 3 h later the activity of 8-aminolevulinic acid dehydratase (ALAD) in the blood, and aspartate aminotransferase (AST), alanine aminotransferase (ALT), γ-glutamyltranspeptidase The activity of blood ALAD was significantly increased by the administration of DMSA and SAM while DMPS had only a moderate effect. The activities of other hepatic enzymes changed little little the the mice were treated with these chelating agents, except for a significant reduction in hepatic ALP activlty following DMPS administration. Arsenic (Ⅲ) administration markedly increased the activities of ALT and ALP in the liver and kidneys. The changes in the en zymatic activities by treatment with arsenic were prevented by injection of DMSA, DMPS and SAM, DMSA being the most effective. These results indicate that DMSA, DMPS and SAM were not toxic to the liver or kidneys of mice and that treatment with DMSA is more effective than DMPS or SAM in protecting mice from acute hepatic or renal toxicity caused by arsenic.