Male Wistar (Porton) rats were dosed by gavage with trimethyltin chloride in arachis oil at doses of 3, 6 and 10 mg/kg. Water consumption and urine production were monitored for 3 days after dosing and at the end of this period the rats were killed and the kidneys were examined histologically. Within 6 h of dosing there was an increase in urine production and in the pH of the urine in all treated groups. Within 24 h of dosing there was an increase in water consumption. Histopathological examination of the kidney at 3 days post-treatment showed nephrotoxic changes ranging from slight vacuolisation of the proximal tubular cells with loss of the brush borders in the 3 mg/kg group to extensive vacuolar degeneration with tubular dilation and evidence of cellular regeneration in the 10 mg/kg group. The results suggest that trimethyltin causes a rapid kidney dysfunction in rats by cytotoxic action on the cells of the proximal tubular epithelium and that this action is dose-related.