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This study documented gastric pH and volume and the number of patients at risk of acid aspiration of gastric contents in a group of mothers undergoing emergency cesarean section under general anesthesia. Patients were randomized in a double-blind fashion to receive ranitidine, 50 mg intravenously, or placebo at the time of decision to proceed to cesarean section. In addition, all patients received 30 mL of 0.3 M sodium citrate on entry into the operating room. Aspiration of gastric contents was undertaken immediately after endotracheal intubation (PI) and before tracheal extubation. Patients with both pH < 3.5 and volume > 25 mL were deemed to be at risk of acid aspiration should regurgitation occur. Postintubation, 12 patients (4%) were at risk in the citrate-alone group and 7 patients (2.3%) were at risk in the ranitidine/citrate group (not significant). Preextubation, 17 patients (5.6%) were at risk in the citrate-alone group and 1 patient (0.3%) was at risk in the ranitidine/citrate group (P < 0.05). PI pH in patients receiving ranitidine/citrate (mean 5.2, sd 0.8) was significantly higher than in patients receiving citrate alone (mean 4.9, sd 1.1). None of the patients who received ranitidine more than 30 min before the PI sample were at risk compared to 6 (3.2%) in the citrate alone group (P = 0.05). We conclude that 50 mg of intravenous ranitidine given at the time of decision to proceed to cesarean section reduces the risk of acid aspiration provided that at least 30 min have elapsed from injection to induction of anesthesia.