We studied effects of selective proximal vagotomy on food-stimulated acid secretion and gastrin release in 7 duodenal ulcer patients. Food-stimulated acid secretion was evaluated by sham feeding patients and by infusing food directly into their stomachs. Vagotomy reduced sham feeding-stimulated acid secretion from 28.2 +/- 4.6 to 1.2 +/- 0.7 meq/hr (95% reduction) whereas infused food-stimulated secretion was decreased from 36.1 +/- 4.6 to 17.9 +/- 5.5 meq/hr (50% reduction). In contrast to the reductions in acid secretion, the gastrin response to infused food doubled after surgery. Although selective proximal vagotomy reduced the rate of acid secretion in response to infused food and also reduced by 64% the peak secretory capacity (peak acid output to pentagastrin), fractional secretion (i.e., the secretion rate in response to infused food expressed as a percentage of the peak secretory capacity) increased significantly after vagotomy from 63 +/- 7% to 91 +/- 11%. This increased fractional secretion in response to infused food was probably a result of exaggerated gastrin release after vagotomy.