Gastric secretory function was studied for 24 hr. In rats bearing permanent fistulas and infused with saline or histamine. Adult normal and thiouracil-treated rats were used in this experiment. Hypothyroidism did produce the depression of gastric secretion, already apparent in saline infused rats but highly significant in histamine infused animals. The absence of normal post-histaminic increase of gastric secretory function in thiouracil-treated rats is considered due to thyroid deficiency.