The effect of the new H2 receptor antagonist ranitidine on gastric pH was studied using a double-blind technique in 36 patients undergoing elective surgery, 18 of whom were given 150 mg of ranitidine orally the night before and on the morning of surgery. The incidence of gastric residue pH higher than 2.5 (p less than 0.01) was significantly greater in patients given ranitidine than in 18 untreated control patients. The mean volume of gastric aspirate in the treated group as 6.7 ml (range 2 to 20 ml) compared with the control group 15.6 ml (range 2 to 44 ml). The higher potency, longer duration of action, and fewer side effects of ranitidine compared with cimetidine suggest that this drug may have clinical advantages over other H2 receptor antagonists.