Gastric pH and volume were measured in four groups of 15 patients scheduled for elective surgery. The patients were randomly allocated to receive either no antacid, oral omeprazole 40 mg the evening before surgery, oral omeprazole 40 mg 2 h before surgery, or effervescent cimetidine 800 mg, 2 h before surgery. Anaesthesia was induced with thiopentone (4-6 mg kg-1), fentanyl (0.03 mg kg-1) and vecuronium (0.1 mg kg-1) and maintained with nitrous oxide in oxygen (50/50) and isoflurane. After induction of anaesthesia and on completion of surgery, gastric pH (mean +/- SEM) and volume were measured using a glass electrode and a phenol red dilution technique. Gastric pH were significantly higher in the three treated groups than in control (P < 0.01). Omeprazole and cimetidine were equally effective in reducing volume and pH of the gastric juice at the beginning and at the end of anaesthesia. Nevertheless 14.2 and 28.5% of patients who received omeprazole respectively the day before or the morning of surgery remained at risk of aspiration pneumonitis (gastric pH < 2.5 and gastric volume > 25 ml).