The immediate (0 to 24 hours) and long-term (4 weeks) hypotensive effects of a new long-acting angiotensin converting enzyme inhibitor, ramipril (HOE 498), as well as adverse effects and tolerance, were evaluated in 34 patients with primary hypertension. Further, effects on serum and urinary aldosterone and circulating angiotensin II concentrations were measured. After short- and long-term administration of 5 or 10 mg of ramipril, the mean blood pressure was significantly lowered compared with placebo. The mean maximum decrease in blood pressure was noted 4 to 8 hours after administration of ramipril once daily. Sustained blood pressure reduction was achieved after 4 weeks of treatment. Serum concentrations of aldosterone and plasma levels of circulating angiotensin II were reduced for up to 12 hours after drug intake, and tended to return to pretreatment levels at 24 hours. Serum angiotensin converting enzyme activity was markedly suppressed for more than 24 hours after a single dose of 5 or 10 mg ramipril. No subjective or objective adverse effects were noted, and the tolerance to the drug was very good.