Captopril is a sulfhydryl-containing analog of proline with antihypertensive activity and potential antineoplastic activity. Captopril competitively inhibits angiotensin converting enzyme (ACE), thereby decreasing levels of angiotensin II, increasing plasma renin activity, and decreasing aldosterone secretion. This agent may also inhibit tumor angiogenesis by inhibiting endothelial cell matrix metalloproteinases (MMPs) and endothelial cell migration. Captopril may also exhibit antineoplastic activity independent of effects on tumor angiogenesis. (NCI04)