The effects of captopril, rentiapril, and alacepril were compared in five patients with renovascular hypertension (RVH) and five with essential hypertension (EH). The dose of each drug was equivalent to 50 mg of captopril. Rentiapril inhibited angiotensin converting enzyme activity more strongly and for longer periods than did the other two drugs. In the patients with RVH, blood pressure was significantly reduced more quickly and for longer periods by rentiapril than by captopril or alacepril; in the patients with EH, alacepril was the most potent antihypertensive agent. The results indicate that rentiapril is as effective as the other two drugs in patients with hypertension and that alacepril is more effective in patients with low-renin hypertension or renin-independent hypertension than captopril or rentiapril.