Dogs with both gastric fistulas of the main stomach and Heidenhain pouches were studied before and after transthoracic truncal vagotomy. Confirming earlier studies in dogs with gastric fistulas alone, truncal vagotomy abolished the acid secretion in response to 2-deoxy-D-glucose (2-DG) and depressed the response to submaximal doses of exogenous gastrin or histamine but did not alter the maximal responses to gastrin or histamine. Effects of truncal vagotomy on acid secretion from the Heidenhain pouches: a) response to 2-DG with the gastric fistulas open was abolished, b) submaximal, but not maximal, responses to histamine were decreased, c) submaximal and maximal responses to exogenous gastrin were markedly increased, and d) response to feeding was markedly increased.