Endogenous hyperparathyroidism was produced in inbred rats and the effect on gastric acid and mucus secretion and mucosal ulceration studied. There was no alteration in gastric acidity except during the period of marked hypercalcemia, when it was slightly decreased. Gastric mucus secretion decreased in 73% of the rats. Twenty-seven percent of the animals developed mucosal ulcerations. In the absence of gastric hypersecretion, it is suggested that decrease in mucus secretion may play a role in the defense mechanism against peptic ulceration in association with hyperparathyroidism.