In vivo and in vitro studies with rats have shown that (14C) niridazole (Ambilhar) binds covalently to tissue proteins, but not to nucleic acids. In the in vitro experiments, binding required the presence of NADPH in the incubation medium, suggesting the production of an active metabolite via a cytochrome P-450-mediated reaction. Niridazole also caused significant dose-dependent decreases in liver and kidney glutathione levels, even though it had no apparent effect on blood glutathione. Alteration of tissue glutathione availability by pretreatment with chloracetamide or cysteine respectively either increased or decreased the NADPH-dependent covalent binding. Pretreatment with phenobarbital, 3-methylcholanthrene or cobaltous chloride, which change the rate of metabolism of (14C) niridazole, similarly altered the extent of protein binding. It is shown that the decrease in tissue glutathione concentration is not due to an effect of the drug on the activities of either glucose-6-phosphate dehydrogenase or glutathione-S-transferases. However, there is a significant reduction in glutathione reductase activity in all the tissues studied. The possible relationships between the results obtained and the cytotoxic effects of niridazole have been discussed.