The effects of oral omeprazole and oral ranitidine on gastric fluid volume and pH were compared in 95 elective surgical patients, randomly assigned to one of three groups. The patients received either 80 mg of omeprazole or 300 mg of ranitidine orally at 6.00 on the morning of surgery. One third of the patients received no antacid therapy. Following induction, a no. 18 nasogastric tube was passed into the stomach and all available gastric fluid was aspirated. pH and volumes were measured. In the omeprazole- and ranitidine-treated groups, the mean pH was > 5.4 after induction, at completion of surgery and 1 h after operation, although at least one patient in both groups had pH < 2.5. The volumes of gastric aspirates were reduced equally by both drugs. Two patients in the omeprazole group, none in the ranitidine group and eight in the control group (26%) had pH < 2.5 with volume > 25 ml at induction. Both drugs appeared to be effective in reducing the volume of intragastric fluid and acidity to acceptable values.