Rats were injected with the herbicide, paraquat dichloride (25 mg/kg, i.p.), and their lungs were perfused 2-28 days later. Isolated lungs from rats treated with paraquat (PQ) 3 or 4 days before perfusion removed significantly less perfused 5-hydroxytryptamine (5-HT) than did saline-injected controls. This effect was not caused by PQ directly, since perfusion of lungs from untreated animals with PQ did not alter removal of co-perfused 5-HT. Monoamine oxidase activity of600 g supernatan fractions of homogenates of lungs from PQ-treated rats was also reduced compared to controls. Although removal of perfused angiotensin I (1 ng/ml) by isolated lungs was not altered by PQpretreatment, antgiotensin-converting enzyme activity in 600 g supernatant fractions of lung homogenates was reducedd significantly. These results suggest that PQ damages pulmonary endothelium and impairs the metabolic function of lung.