Zolpidem is a new imidazopyridine derivative acting as a hypnotic which may be prescribed with H2 receptor antagonists in patients with peptic ulcer. A cross-over study (cimetidine, 1 g daily for 19 days; ranitidine, 300 mg daily for 19 days; wash-out period: 20 days) was carried out in six healthy volunteers. Zolpidem, 20 mg was administered orally at 09h00 prior to any treatment and on days 2 and 17 of each treatment period. Antipyrine clearance was also determined before and on day 18 of each treatment period. Under these experimental conditions, the inhibitory effect of cimetidine on the Cyt P-450 mono-oxygenase system has been demonstrated (reduced clearance of antipyrine, p less than 0.01) but the pharmacokinetics of zolpidem did not appear to be modified. Zolpidem induced hypnotic effects for the first 3 h which tend to be prolonged by the combination with cimetidine. Psychometric and pharmacokinetic evaluations did not show significant interactions with either anti-H2 receptor agent following zolpidem administration.