Pulmonary injury induced by the plant alkaloid monocrotaline is partially prevented by the angiotensin-converting enzyme (ACE) inhibitor captopril. CL242817 [(S-[R*,S*])-1-([3-acetylthio]-3-benzoyl-2-methyl-propionyl)- L-proline] is a new orally active ACE inhibitor under evaluation as an antihypertensive agent. To determine whether CL242817 also can modify monocrotaline-induced pulmonary injury, male rats were divided into four groups: control; CL242817 (60 mg/kg/day, po); monocrotaline (2.4 mg/kg/day, po); or monocrotaline plus CL242817, and were sacrificed after 6 weeks of continuous treatment. Rats receiving monocrotaline alone exhibited occlusive medial thickening of the pulmonary arteries, cardiomegaly, and right ventricular hypertrophy. Electron micrographs of monocrotaline-treated lung revealed degeneration of both endothelial and Type I epithelial cells, as well as marked interstitial hypercellularity and fibrosis. Hydroxyproline (collagen) content of monocrotaline-treated lung also increased significantly, confirming the fibrosis observed in the electron micrographs. These structural changes were accompanied by decreased lung ACE and plasminogen activator (PLA) activities, indicative of pulmonary endothelial dysfunction. Concomitant CL242817 treatment ameliorated all anatomic manifestations of monocrotaline injury, particularly the right ventricular hypertrophy, pulmonary arterial occlusion, epithelial degeneration, and interstitial fibrosis. CL242817 also significantly prevented the monocrotaline-induced increase in lung hydroxyproline content. In contrast, concomitant CL242817 did not significantly influence the suppressed lung ACE and PLA activities in monocrotaline-treated rats. CL242817 alone produced retarded weight gain, decreased heart weight relative to body weight, decreased lung hydroxyproline content and ACE activity, and increased serum ACE activity and plasma AII concentration. Thus CL242817 resembles captopril, both in its ability to ameliorate monocrotaline-induced pulmonary injury in rats, and in many of its side effects.