Aspiration pneumonitis, Mendelson's syndrome, continues to be an important cause of maternal morbidity and mortality in obstetric anaesthesia, despite widespread adoption of the practice of routine administration of alkalis. Histamine H2 receptor blocking drugs have been shown to reduce gastric secretion in non-obstetric patients. Cimetidine was given intravenously to fasting patients before elective Caesarean section. In all 10 patients who received cimetidine 200 mg intravenously at 60--80 minutes before anaesthesia, the pH of gastric contents at the time of induction was above 2.5. When the interval between administration of the drug and induction of anaesthesia was only 30--40 minutes (six patients) or was over 90 minutes (20 patients) then the pH was raised to above this value in only two-thirds of patients. By contrast eight of 10 untreated patients were found to have a gastric pH of less than 2.5. No adverse effects of cimetidine were seen in mothers or infants.