A controlled trial was carried out on 120 healthy patients presenting for elective surgery. The patients were divided randomly into three groups, which received respectively, 50 mg ranitidine, 100 mg ranitidine, or 300 mg cimetidine intramuscularly at least 45 minutes before operation. Following induction of anaesthesia, a nasogastric tube was passed, the stomach contents aspirated and analysed for volume and pH. Thirteen per cent of cimetidine-treated patients had a gastric pH of 2.5 or less, compared to 8% of those given ranitidine 50 mg and 3% of those given ranitidine 100 mg; however, these differences were not statistically significant. No side effects attributable to the administration of either drug were observed. It is concluded that intramuscular administration of ranitidine or cimetidine is an effective method of reducing the number of patients at risk of acid aspiration during anaesthesia. However, neither drug eliminates the risk of acid aspiration in all patients, and thus careful anaesthetic technique to protect the airway remains essential.