The authors evaluated the applicability of the rat with gastric fistula to the bioassay of gastrone, endogenous inhibitor of gastric acid secretion and the role of anti-cholinergic mechanism in this inhibition. 104 individual bioassays were performed on 25 rats with chronic gastric fistula out of which 34 tests were performed with gastrone, 49 controls with saline and 21 with atropine. In rats serving as their own controls, a statistically significant reduction of the volume of basal gastric secretion and of the basal HCl output by 56 and 59 %, respectively, occurred after intravenous administration of 40 μg gastrone B. This was demonstrated in the first 4 h after gastrone and was statistically highly significant (p