This study was undertaken to investigate the effect of the acidosis on the gastric acid secretion in the isolated whole stomach of the rat and the effect of prostaglandin E₁ on the gastric acid secretion influenced by the acidosis. Twenty-two male albino rats(Sprague-Dawley strain) were used. The isolated whole stomach from each rat was introduced into the Kreb s solution which was continuously gassed with 95%O₂-5%CO₂ for 1 hour, after irrigation of the lumen with cold physiological saline(4℃). Thereafter, each stomach was irrigated again with 5% dextrose solution (pH 7.4, 37℃), and filled with the dextrose solution. All the stomachs with the dextrose solution were divided into 4 groups according to the Kreb s solutions in which each stomach was incubated for 30 min: 1) control group, in the pH 7.4 solution, 2) PGE₁ group, in the pH 7.4 solution containing 5 μg/ml of PGE₁, 3) acid group, in the pH 7.0 solution, and 4) acid+PGE₁ group, in the pH 7.0 solution containing 5 μg/ml of PGE₁. After incubatory period, the contents of each stomach were collected and centrifuged(1,500 rpm, room temperature) for 15 min. The acid output in the supernatant was determined with 0.012 N NaOH by means of autotitrator(Dosimat, Metrohm Herisau Co.) at pH 7.4. Results obtained were as follows: 1) The acid output of the acid group increased significantly in comparison with the control value. 2) The acid output of the acid+PGE₁ group decreased significantly in comparison with the acid group. It is inferred from the above results that the acidosis facilitates the gastric acid secretion and PGE₁ inhibits the gastric acid secretion induced by the acidosis.