The role of the vagus nerve in the stimulation of gastric acid secretion was investigated by comparing the acid output in response to gastric distension during augmented secretion mediated by electrical vagal stimulation and pentagastrin. Under control conditions distension increased acid output from 29.8 +/- 3 mumol/10 min to 59.9 +/- 5.5 mumol/10 min (P less than 0.0001). Bilateral cervical vagotomy significantly reduced both basal acid secretion and the response to distension although the increase during distension was still significant. Electrical vagal stimulation augmented basal acid output and the response to distension, and at 1 Hz both parameters were not different from that before vagotomy. However, the sensitivity of the response to distension was unchanged by vagal stimulation. Pentagastrin infusion augmented basal acid secretion and the response to distension in a similar manner to vagal stimulation. We conclude that the vagus nerves potentiate the gastric acid secretory response to distension not by facilitating enteric reflexes but by an action at the level of the parietal cell.