The effect of preanesthetic cimetidine and metoclopramide on gastric contents in inpatients undergoing elective surgery was studied. One hundred and fifty patients were allocated randomly into six groups with 25 patients in each group. Patients in Group 1 served as control. Group 2 patients received metoclopramide in the morning. Group 3 patients received cimetidine at bedtime and in the morning. Patients in Group 4 received cimetidine at bedtime and metoclopramide in the morning. Group 5 patients received cimetidine and metoclopramide in the morning, while patients in Group 6 received cimetidine at bedtime and metoclopramide and cimetidine in the morning. Cimetidine 300 mg and metoclopramide 10 mg were administered by mouth with a sip of water at bedtime or in the morning 1-4 h prior to the induction of anesthesia. Patients with gastric pH less than or equal to 25 or gastric content volume greater than or equal to 25 ml were defined to be at risk of pulmonary damage if aspiration should occur. In the control group the mean pH and volume of gastric contents were 2.89 and 22.3 ml, respectively, with 64% of patients with pH less than or equal to 2.5 and 32% of patients with volumes of greater than or equal to 25 ml. Cimetidine and metoclopramide favorably modified the risk factors in all the experimental groups. This study demonstrated that the three groups receiving cimetidine in the morning (Groups 3, 5, and 6) had significantly greater mean gastric pH than the other groups. Gastric volumes were significantly less in all experimental groups.(ABSTRACT TRUNCATED AT 250 WORDS)