Effects of cytochalasin E on the secretion of H+ and volume of gastric juice were investigated in gastric fistula rats. Direct exposure of the gastric mucosa to cytochalasin E (5-20 micrograms/ml) inhibited H+ secretory rate in all of the pentagastrin-, histamine- as well as non-stimulated rats but it did not affect the secretory volume of gastric juice. The inhibitory action of cytochalasin E on H+ secretion had a rapid onset and showed a dose-dependent pattern. The mechanism of inhibitory action was not related to its interaction with plasma membrane receptors or membrane enzyme, K+-stimulated ATPase. Interference with cellular energy production as well as alteration of microfilament function, which are essential for H+ production and secretion respectively, are likely to be the bases of the cytochalasin inhibition.