The effect of cimetidine on the disposition kinetics of phenytoin was investigated in 7 healthy volunteers. Each subject received a single intravenous dose of phenytoin on two occasions, in the control state, and during concurrent treatment with cimetidine 1 200 mg/day for 6 days. A slight but statistically significant decrease both in the rate of elimination and total body clearance of phenytoin was observed during the administration of cimetidine. The effect is probably due to inhibition of metabolism.