Single doses of 100, 200, 300, and 400 mg diquat [herbicide] ion/kg body wt were administered by stomach tube to cynomolgus monkeys. Within 4 days of dosing, deaths occurred in 1/2, 0/4, 1/2, and 2/2 monkeys, respectively. All animals developed diarrhea and the most severely affected became comatose. The most important histopathological changes were necrosis of the epithelium and villi of the gastrointestinal tract, and the epithelium of the proximal and distal convoluted tubules of the kidneys.