Smooth muscle cell proliferation and formation of extracellular matrix are parts of the repair process after vascular injury. Similar processes occur after coronary angioplasty and, in approximately 33% of vessels, lead to intimal hyperplasia and vascular restenosis within 6 months after angioplasty. In a rat model of balloon catheterization, the proliferative response to balloon injury was reduced by 70% and the area of vascular wall covered by lesion formation was decreased by 45% in rats treated with the angiotensin-converting enzyme inhibitor cilazapril. Other antihypertensive agents were much less active when tested for suppression of intimal hyperplasia after balloon injury: verapamil 0%, minoxidil 4% and hydralazine 34%. For cilazapril at the dose of 10 mg/kg per day, approximately 20% greater suppression of intimal hyperplasia was seen when the treatment was started 6 days before balloon injury. Treatment of rats fron the time of balloon catheterization with both cilazapril (10 mg/kg per day) and heparin infusion (0.3 mg/kg per h) resulted in essentially complete (> 90%) inhibition of intimal hyperplasia. These data indicate that the angiotensin-converting enzyme inhibitor cilazapril specifically inhibits the proliferative response to balloon injury and that heparin and cilazapril inhibit intimal hyperplasia through different mechanisms. The data also suggest that the use of pharmacologic combinations may have therapeutic usefulness to prevent late restenosis after coronary angioplasty.