1. Perindopril, an orally active angiotensin converting enzyme inhibitor, was given to 23 hypertensive patients with stable chronic renal failure for 15 days. The dose of perindopril was 2 or 4 mg once a day according to the degree of renal failure. The creatinine clearance of the patients ranged from 6 to 67 ml min-1 1.73 m-2. The pharmacokinetics of perindopril and perindoprilat, its active metabolite, were studied after acute and chronic administration of perindopril. 2. The drug was well tolerated and creatinine clearance was unaltered by treatment. 3. In both groups, steady-state was reached within 3 days of chronic treatment. 4. After both acute and chronic drug administration renal impairment had no effect on perindopril pharmacokinetics but the pharmacokinetics of perindoprilat were altered significantly. After chronic administration the serum accumulation ratio was 1.81 in patients with mild renal failure and 5.35 in patients with severe renal failure. Chronic administration did not modify the renal clearance of perindoprilat nor its elimination half-life. 5. A significant correlation between the renal clearance of perindoprilat and creatinine clearance was observed (r = 0.87 first dose, r = 0.83 last chronic dose). 6. A non-linear relationship between serum perindoprilat concentration and inhibition of angiotensin converting enzyme was described by a modified Hill equation. Values of IC50 were 1.11 +/- 0.07 micrograms I-1 (mean +/- s.d.) in patients with severe renal failure and 1.81 +/- 0.20 micrograms l-1 in patients with moderate renal failure. Chronic administration increased maximal inhibition and decreased the time to maximal inhibition only in patients with severe renal failure.(ABSTRACT TRUNCATED AT 250 WORDS)