Blood flow distribution to the mucosa-submucosa and to the muscularis in the cat small intestine was investigated with a 85Kr elimination technique before and after exposing the intestinal mucosa for 30 min to cholera enterotoxin. In all experiments the toxin induced an intestinal secretion. Concomitantly, total intestinal blood flow was increased to a level 50 per cent above control 3 h after exposure. This vasodilatation reflected a doubling of mean blood flow in the mucosa--submucosa while muscularis blood flow remained unchanged. In another series of experiments the effect of cholera toxin on intestinal capillary hydraulic conductivity was investigated by determining the capillary filtration coefficient (CFC). A slight increase in CFC was noted during the 3 h observation period but this was not more pronounced than would have been expected from the concomitant vasodilatation. It is concluded that hemodynamic changes in the intestinal mucosa may be one of the several factors that probably are involved in the pathogenesis of cholera.