The purpose of this study was to characterize and compare the toxicity, clinical syndrome, and pathology of paraquat in rats, guinea pigs, and monkeys. The LD 50 of oral paraquat was 126, 22, and 50 mg/kg in the three species, respectively. Clinical signs of intoxication were anorexia, adipsia, diarrhea, hyperpnea, dyspnea, and tachycardia. All animals were hypoxic. Monkeys which received greater than 63 mg/kg of paraquat died within 1–2 days. Death was preceded by convulsive seizures. Monkeys that received paraquat dosages of 50–53 mg/kg showed signs of dyspnea prior to death while those monkeys given 30–40 mg/kg paraquat showed signs of dyspnea for 1–5 days and subsequently developed pulmonary fibrosis. Focal necrosis was noted in liver, kidney, and the gastrointestinal tract but the primary lesion occurred in the lungs of all species. Animals that died in less than 7 days showed pulmonary hemorrhage, edema, and congestion. Rats and monkeys but not guinea pigs developed interstitial fibrosis of the lung after 7–10 days. Monkeys did not have liver and kidney dysfunction. It was concluded that the paraquat-induced pulmonary fibrosis in humans could be duplicated in monkeys and rats.