It has recently been shown that distension of the stomach in anesthetized pigs causes reflex hemodynamic responses through efferent sympathetic mechanisms. The present study was undertaken to investigate whether these mechanisms include activation of the renin-angiotensin system. In twelve anesthetized pigs, intragastric balloons were distended for periods of 30 minutes by 0.81 of warm Ringer solution (mean gastric transmural pressure of about 12 mmHg). Changes in arterial blood pressure and heart rate were respectively prevented by a pressurized reservoir connected to the left femoral artery and by atrial pacing. Plasma renin activity was measured during the last minute of distension by radioimmunoassay of angiotensin I. In each of the twelve pigs distension of the stomach caused an increase in plasma renin activity. In five pigs, this response was graded with step increments of the distension. The increase in plasma renin activity to gastric distension was abolished by bilateral subdiaphragmatic vagotomy (six pigs) and by bilateral section of the renal nerves (six pigs). The present study showed that innocuous distension of the stomach in the anesthetized pig reflexly increased plasma renin activity. The afferent limb of the reflex was in the vagal nerves and the efferent limb involved renal nerves.