This study was undertaken to compare the potency of L- and D-isomers of natural amino acids (AA's) infused intravenously for stimulation of gastric acid secretion in 3 dogs with Heidenhain pouches (HP) and gastric fistulae. L-Isomers of all natural AA's were found to stimulate acid secretion from the HP, whereas D-isomers were significantly less effective. The most potent L-isomers of AA's were histidine, phenylalanine, glycine, tryptophan, and alanine, which caused an increase in acid output reaching, respectively, 63, 45, 42, 39, and 33% of the maximal response to histamine. The stimulation of acid secretion was not accompanied by any significant change in serum gastrin level. Distention of the HP during intravenous infusion of L-histidine or L-phenylalanie solution caused a pressure-related increase in acid output reaching a peak at 30 cm distention pressure. Decreasing the luminal pH of the HP in sequential order from 7.0 to 2.5 resulted in a stepwise reduction of the HP response to intravenous histidine or phenylalanine, falling at pH 2.5 to about 20% of the peak response achieved at pH 7.0. Metiamide caused a profound reduction of histidine but had only a slight effect on acid secretion induced by intravenous infusion of other AA's suggesting that histidine excites the oxyntic cells mainly through the transformation to histamine and activation of H2-receptors. Atropine also suppressed gastric acid secretion stimulated by intravenous AA infusion, suggesting a role of a cholinergic mechanism in this stimulation. We conclude that L- and, to a lesser degree, D-isomers of natural AA's infused intravenously cause stimulation of gastric acid secretion by a gastrin-independent mechanism sensitive to distention pressure and pH of gastric content.