Cimetidine 10 mg/kg orally was given at varying times from 60 to 240 minutes pre-operatively to 100 healthy children between the ages of 6 months and 14 years. Cimetidine proved to be most effective when given between 120 and 180 minutes before the induction of anaesthesia. All patients in this group had a gastric pH of more than 2.5 and the mean volume aspirated was also significantly lower than that in the control group. The average peak blood concentration after 10 mg/kg oral cimetidine in four healthy children was 3.25 micrograms/ml (range 1.20-4.80) and occurred 75 minutes after administration (range 60-120 minutes). In these patients the mean (SD) half-life of cimetidine was 138 (18) minutes. The reduction of gastric juice volume and acidity produced by 10 mg/kg oral cimetidine given 120-180 minutes prior to induction of anaesthesia has important clinical implications.