Gastric fluid pH was measured immediately after anesthetic induction in 150 fasted adult patients with or without prior cimetidine. All patients had received morphine-atropine preanesthetic medication. Gastric fluid pH was above 2.5 in 39/50 patients (78%) receiving cimetidine 300 mg orally with 50 ml H2O the evening before operation. The same dose of oral cimetidine but 60 to 90 minutes before anesthetic induction resulted in gastric fluid pH above 2.5 in 42/50 patients (84%). In contrast, gastric fluid pH was above 2.5 in only 20/50 patients (40%) not receiving cimetidine. We conclude that oral cimetidine administered 60 to 90 minutes before anesthetic induction is a practical way to increase gastric fluid pH above 2.5 in the majority of fasting adult patients.