The isolated whole stomach of the mouse was used to study the effect of atropine on gastric acid secretion stimulated by acetylcholine, bethanechol and histamine. Atropine inhibited acid secretory response to acetylcholine and bethanechol in a competitive manner. The following pA2 values for atropine were calculated: 6.72 and 7.75, respectively. Slopes of log (DR-1) against log (B) were not significantly different from unity. Histamine-induced acid secretion was not inhibited by atropine. Acid secretory response to acetylcholine was not reduced by concentrations of cimetidine up to 10-3 mol/l. Acetylcholine-induced acid secretion was not influenced by hexamethonium. In the mouse isolated stomach, acetylcholine stimulates acid secretion through a direct cholinergic pathway.