The dose-response effects of intravenous ranitidine given 45min to 5 hearlier on gastricpH and volume were evaluated in six groups of 25 outpatients, each undergoing elective surgery under general anesthesia. Patients in Group 1 received no ranitidine and served as controls. Patients in Groups 2–6 received ranitidine intravenously in incremental doses of 0.5 mg · kg−1body weight from 0.5 mg to 2.5 mg (Group 2, 0.5 mg; Group 3, 1.0 mg; Group 4, 1.5 mg; Group 5, 2.0 mg; and Group 6, 2.5 mg). Ninety-six per cent of patients in the control group (Group 1) had gastricpH ≤ 2.5 while 36% of the patients had gastric content volumes ≥ 25 ml withpH ≤ 2.5. Ranitidine, in incremental doses of 05.-2.5 mg · kg−1body weight, caused a significant reduction of gastric acidity and volume. The ED30of ranitidine producing a gastricpH ≥ 2.5 was 0.36 mg · kg−1, and the ED95was 0.98 mg · kg−1body weight. The ED95of ranitidine producing a gastric volume ≤ 25 ml was 1.96 mg · kg−1. At the dose of 1.5 mg · kg−1of ranitidine, 100% of the patients had gastric contents withpH ≥ 2.5. The proportion of patients with volume ≤ 25 ml was 68% with ranitidine, 0.5 mg · kg−1, and gradually increased to 100% with 2.5 mg · kg−1body weight. It is concluded that a significant number of outpatients are at risk for aspiration of acid gastric contents and that this risk is lowered by preoperative administration of ranitidine.