Surgery patients are considered at risk for pulmonary acid aspiration if the gastric pH is 2.5 or less and/or the gastric aspirate volume is 25 ml or more. This study compares the effect of cimetidine, magnesium tri-silicate (MTS) and metoclopramide on the intragastric volume and pH in 120 elective surgery patients divided into four equal groups. Sixty to ninety minutes before anaesthesia, the cimetidine and metoclopramide groups received 300 and 10 mg i.m., respectively. One hundred twenty minutes before anaesthesia, the MTS group received 30 ml orally. Patients receiving MTS showed the highest gastric pH and intragastric volume. The cimetidine group had the lowest intragastric volume and a significant increase in pH. The metoclopramide group effectively reduced gastric volume but showed no significant effect on pH. We conclude that no single drug can be relied upon to reduce intragastric volume and neutralize gastric acid in all patients.