A number of substances (" gastrones ") found in gastro-intestinal mucous secretions inhibit gastric acid secretion when given by intravenous injection in vivo. The mode of this action is unknown. The present experiments have defined the effect of two potent gastrones on acid secretion by isolated rat gastric mucosa. In each instance the addition of the gastronE preparation to the solution bathing the in-vitro mucosal preparation resulted in significant extra secretion of acid, although the same gastrone preparations had inhibited acid secretion in the intact animal. This paradoxical stimulant effect of gastrone on isolated gastric mucosa has not previously been described. Its demonstration is evidence that the inhibitory action of these substances in the intact animal is exerted at a more proximal site than the parietal cell.