The hemodynamic effects of systemic blood pressure of varying intraluminal pressure in the celiac and superior mesenteric arteries through a range of 20–210 mm Hg were examined in dogs. Responses were compared to a smaller series of cats. Systemic arterial pressure in the dogs rose and fell only about 10% from the mean through this range of decrease and increase, respectively, in the splanchnic arteries. By comparison, comparable decrements of pressure in the celiac and mesenteric arteries of cats gave marked increases in pressure averaging 74%. It was found that the sino-aortic reflexes held the splanchnic mechanism in check in the dog for with sinus denervation the response was significantly increased. Evidence indicated that the responses could be explained on a hydrodynamic basis and that splanchnic baroreceptor reflex activity in the dog is slight.