Light and electron microscope studies in ligated rabbit gut loops showed that cholera vibrios El Tor attached themselves to the enterocytes and multiplied, most probably on the intestinal epithelium surface. But they did not penetrate deeper and were unable to cause destruction of the epithelium or marked inflammation. Ultrastructural manifestations of both apocrine and merocrine secretion of the enterocytes were observed. In addition, the enterocytes showed signs of generally enhanced activity. Some other changes and alterations in the lamina propria, presumably induced by excessive transport of water, are also described. The enterotoxin-induced epithelial hypersecretion with fluid accumulation in the gut loops resulted in the attached vibrios being cleared off the epithelium lining into the gut lumen. NAG vibrios were mostly incapable of inducing the above changes. We discuss the importance of cholera vibrio attachment to and its multiplication on the intestinal epithelium in the pathogenesis of cholera.