25 patients with different degrees of chronic stable renal failure received oral treatment with cimetidine over 6 days and a final dose in the morning of day 7. The doses of cimetidine were reduced according to the degree of renal failure. Plasma concentrations of cimetidine were determined before the morning dose on days 2,3,6 and before the final morning dose and during the day on day 7. 24-hour excretions of cimetidine were measured during the study and on day 7, 6 to 9 hours after the final morning dose. There were significant linear relationships between the values of creatinine clearance and plasma elimination rate constant of cimetidine (p less than 0.0025) and between the values of creatinine clearance and renal clearance of cimetidine (p less than 0.05). On the basis of the morning plasma concentrations before and the plasma concentration curve after the final morning dose on day 7 the following dose recommendations according to pretrial values of creatinine clearance are recommended for cimetidine treatment in renal failure: 5 to 15ml/min 200mg 2 or 3 times daily, 15 to 30ml/min 200mg 3 times daily and 30 to 75ml/min 200mg 4 times daily.