1. Lisinopril, a new orally active angiotensin converting enzyme inhibitor, was given to eight patients with stable chronic renal failure, in a dose of 5 mg 24 h-1 for 1 week. Creatinine clearance of the subjects ranged from 0.22 to 1.11 ml s-1. Lisinopril pharmacokinetics were studied over 8 days. 2. There was a close correlation between creatinine clearance and total 'area under the curve' over the 8 days of study (r = -0.88, P less than 0.05), and plateau lisinopril concentration and creatinine clearance (r = -0.77, P less than 0.05). 3. Serum angiotensin converting enzyme activity was inhibited in proportion to log serum lisinopril concentration (r = -0.99, P less than 0.001). Calculated IC50 was 47 ng lisinopril ml-1. from pooled data, with individual patients IC50 ranging from 20 to 70 ng lisinopril ml-1. 4. Creatinine clearance was unaltered by treatment. Serum potassium rose to over 5 mmol 1-1 in four patients, without adverse clinical effect.