The binding and internalization of cholera toxin (CT) into the intestinal epithelium were studied in vivo in rats. The distribution of CT was ascertained using immunofluorescence and by immunoenzyme-electron microscopy, with horse-radish peroxidase anti-CT antibodies as the conjugate. The toxin was rapidly bound and internalized into both epithelial and goblet cells; CT was evenly distributed on the microvilli at the bases of which it appeared in invaginations (coated pits). Though not found in nuclei, CT appeared intracellularly in coated vesicles, and dissolved in the cytoplasm where it was enriched at the terminal web. The basolateral membrane, except for the tight junctions, was outlined with CT; some staining also appeared in the basement membrane, in fibroblasts, macrophages and in the blood-vessel walls in the submucosa. The lysosomatotrophic agent chloroquine simultaneously inhibited CT-induced fluid secretion and intracellular distribution of CT in the cytoplasmic matrix, but not in the vesicles. The inhibitor of CT-action on adenylate cyclase, chlorpromazine, did not affect the cellular distribution of CT. Our results suggest that CT mainly is internalized by endocytosis into the intestinal epithelium. The toxin is probably released from vesicles into the cytoplasm via secondary lysosomes.