The role of the autonomic nervous system in the generation mechanism of gastric stress ulcers has long been attracting attention. We have been investigating the ulcer formation in the spontaneously hypertensive rat (SHR) thought to have a functionally facilitated sympathetic nervous system and in the MGS rat in which the function of the sympathetic nervous system may be attenuated. In the present study, we studied the state of gastric acid secretion under a pyloric ligation condition in these rat species, using WKY rats as the control. The pylorus was ligated for 4 hrs, and the volume and pH of gastric juice and the volume of gastric acid secreted were measured. Both SHRs and MSG rats showed higher pH values and smaller volume of secreted acid than WKY rats. Ulcer formation in SHRs was milder than in control rats as reported previously, and this was thought to be related to the high pH value and the low gastric acid secretion observed in the present study. Although ulcer formation in MSG rats was more frequent than in control animals, factors such as mucosal blood flow etc. were suggested to be rather responsible because of the high pH value and low acid secretion in this species