The influence of vagotomy on bicarbonate secretion in the proximal duodenum was assessed in rats receiving truncal, hepatoduodenal and gastric vagotomy. Two weeks after vagotomy, animals fasted for 24 hr were subjected to laparotomy under urethane anesthesia. A proximal duodenal loop excluding the outlet of the common bile duct was prepared as part of an airtight extracorporeal perfusion circuit. The circuit was filled with a bathing solution at pH 4.5 and which was perfused at a rate of 10 ml/min at a constant temperature of 37 degrees C. The pH and CO2 tension of the bathing solution were determined using microelectrodes, and bicarbonate secretion was calculated from the pH and CO2 tension data by the Henderson-Hasselbalch equation. The bicarbonate secretion at the first portion of the duodenum over a 60-min period was significantly higher in the truncal and hepatoduodenal vagotomy groups than in the control group. However, there was no significant difference between the gastric vagotomy and control groups. Furthermore, no significant difference was noted between the truncal and hepatoduodenal vagotomy groups. Since bicarbonate secretion was increased by hepatoduodenal vagotomy, the hepatoduodenal branch of the vagus nerve appeared to inhibit bicarbonate secretion at the first portion of the duodenum.