Suckling mice aged 4-5 days were injected with Vibrio cholerae hemagglutinin/protease and ultrastructural changes in their small intestine were studied after 5 h. The preparation caused a statistically significant accumulation of fluid in the intestine, appearance of large gaps along cell-cell spaces in the villi and crypts, intense production and secretion of the mucus by goblet cells. The formation of interepithelial cavities was paralleled by vascular changes, supplemented by extravasal disorders caused by mast cell reaction. The role of enterochromaffin cells and lipofibroblasts, modulating the secretion in the intestine, is confirmed.