The pharmacokinetics and bioavailability of cimetidine after 200 mg given intravenously and orally was studied in 6 gastric and 6 duodenal ulcer patients aged between 28 and 64 years. There were no differences in any of the pharmacokinetic parameters between the two ulcer groups, but there was considerable variation which was mainly age-related. Bioavailability was not complete, but was about 60% based on the plasma concentration data. The volume of distribution at steady-state was about 80% of body weight, and the plasma clearance value of 495 ml/min was mainly due to renal clearance (293 ml/min). The elimination half-life was approximately 2 hours. These pharmacokinetic parameters together with the percentage of dose excreted unchanged in urine following intravenous administration were all highly age correlated; all decreasing with age except the elimination half-life which increased with age. Of clinical importance is the finding that the time for which the plasma concentration exceeded 0.5 microgram/ml following oral administration was also highly age-dependent, increasing by more than 2-fold in the elderly (53 to 64y) compared with the younger (28 to 45y) patients.