This placebo-controlled trial compared the effects of preoperative, intravenous cimetidine (300 mg) or ranitidine (50 mg) on gastric pH and gastric volume in 31 adult patients requiring general anesthesia. The elapsed time from drug administration to initial gastric sampling did not differ significantly between ranitidine (45 minutes), cimetidine (48 minutes), or placebo (52 minutes) treated patients. Ranitidine, but not cimetidine, significantly (P = 0.02) increased gastric pH when compared with placebo. Gastric pH correlated (r = 0.7, P = 0.01) with cimetidine concentration in gastric fluid at induction. Gastric pH was directly proportional to ranitidine concentration in gastric fluid at induction, but the correlation was weak (r = 0.54, P = 0.1). The H2 blockers did not significantly alter gastric volume when compared with placebo. The number of patients with gastric pH less than = 2.5 and gastric volume = greater than 25 ml did not differ significantly between cimetidine (8%), ranitidine (10%), and placebo (22%). No clinical evidence of aspiration pneumonitis was found in our study patients.