We have compared the effects of single oral doses of omeprazole 40 mg, famotidine 40 mg or placebo on gastric secretion in 45 non-obese patients the night before elective biliary surgery. After stable anaesthesia had been established, a Salem orogastric tube was introduced and gastric contents were aspirated by a blinded observer. The volume and pH were noted. After the abdomen was opened aspiration was repeated but on this occasion with the surgeon's manual assistance. We found that the initial aspirate volume underestimated total gastric volume by an average (SD) of 7.1 (6.6) ml. Famotidine, but not omeprazole, produced a significant decrease in gastric volume and acidity. Patients were considered to be at risk if pH < 2.5 and volume > 0.4 ml.kg-1. Three patients in the omeprazole group, three in the placebo group and none in the famotidine group came into this category. We conclude that a single oral dose of omeprazole 40 mg given the night before surgery does not afford adequate prophylaxis for acid aspiration syndrome.