The pharmacokinetics of ramipril (HOE 498) were studied after oral administration of a single 10 mg dose to 24 hypertensive patients with different degrees of renal function. The creatinine clearance ranged between 4.1 and 126 ml/min/1.73 m2 and was below 35 ml/min/1.73 m2 in 16 patients. Angiotensin converting enzyme activity and the concentrations of ramipril and its active diacid metabolite ramiprilat were measured in plasma up to 10 days after drug intake. Urine levels of ramipril, ramiprilat, their glucuronides and 2 major metabolites (a diketopiperazine and a diketopiperazine acid) were measured up to 4 days after medication. The plasma concentration-time curve of ramiprilat was polyphasic with an initial steep decline after the peak level and a subsequent very long terminal phase at low concentrations. Impaired renal function resulted in higher peak levels of ramiprilat, longer times to peak and a markedly slower decline of plasma ramiprilat levels. Hence, the duration of angiotensin converting enzyme inhibition was considerably prolonged in renal failure and depended on the severity of renal impairment. The urinary excretion of ramipril and its metabolites decreased with decreasing renal function and was linearly related to the creatinine clearance, suggesting an alternative pathway of elimination. The pattern of excretion rates of ramipril and its various metabolites was not affected by renal failure. In contrast to the marked changes in the renal elimination, no relevant differences were observed in the absorption of ramipril from the gastrointestinal tract. Systolic and diastolic blood pressure decreased in all groups. The single 10 mg dose of ramipril was well tolerated.