Abstract A single oral dose of the malathion impurity, O,O,S-trimethyl phosphorothioate (OOS-Me) resulted in a substantial increase in the amount of amino acids excreted in rat urine over a 10-day period as estimated by the ninhydrin method. Levels of amino acids in the urine of treated rats increased as high as 13-fold compared to levels found in untreated rats. The effects of OOS-Me was dose dependent, notable amino aciduria being observed at 20–60 mg/kg and no effect observed at 10 mg/kg. Animals receiving 40 and 60 mg/kg OOS-Me manifested typical signs of delayed toxicity, e.g., weight loss and refusal of food and water. Moreover, the amino acid content in the urine of rats treated with 60 mg/kg OOS-Me adulterated with 3 mg/kg OOO-Me (5% antagonist) was comparable to that found with control rats. In contrast to the urine study, the results reveal essentially no difference in amino acid levels in the blood of control rats and rats treated with the various dosages of OOS-Me. This finding, coupled with evidence of the occurrence of oliguria, diuresis, and reduction in blood urea nitrogen-creatinine ratios, suggests that OOS-Me may cause acute kidney tubular damage.