This study documented gastric fluid acidity in ambulatory surgical patients 30-120 min after they had taken a 300-mg tablet of ranitidine, as changes specific to this relatively brief dose-to-sampling interval previously had not been delineated. At 20-120 min before outpatient surgery, ranitidine was given with 15 mL of water to 112 ASA physical status I-III adults without increased risk factors for aspiration of acidic gastric contents. After induction of general endotracheal anesthesia, the gastric fluid was aspirated. Volume and pH were compared with a reference group (pH = 1.6 +/- 1; n = 161) that did not receive an H2-antagonist or water. Of 40 patients sampled at 30-60 min after dosing, 26 had pH greater than or equal to 2.5; mean pH was 3.9 +/- 2 (P less than 0.05 vs reference by Wilcoxon signed rank and chi 2 tests). Beyond 90 min, 28 of 28 patients had a pH greater than or equal to 2.5, with a mean of 6.3 +/- 1. No significant differences were noted with respect to mean gastric volume. We conclude that in patients without additional risk factors, oral ranitidine consistently prevents acid production when it is administered more than 90 min before induction of anesthesia.