Pentagastrin, histamine, and acetylcholine stimulated acid secretion in the isolated rhesus (Macaca mulatta) gastric mucosa. The potencies were in the order pentagastrin greater than histamine greater than acetylcholine. However, histamine was more effective, the calculated maximal response being twice as great as for the other two agonists. Metiamide inhibited acid secretion induced by both histamine and pentagastrin. Whereas the inhibition of histamine by metiamide was surmountable, that of pentagastrin was not. Atropine inhibited the response to acetylcholine. Even in higher doses, atropine had no effect on histamine. With stimulation, transmucosal potential differences and resistances decreased; the changes were reversed on inhibition.