The human intestinal tract, as well as that of several eutherian and metatherian mammals, was examined for the distribution of heat-stable enterotoxin (ST)/guanylin receptors. These receptors were confined to the intestinal epithelium lining the lumen and forming the intestinal glands throughout the length of both the small intestine and colon of all species examined. In man and most other mammalian species, there appeared to be a decrease in receptor density distally along the longitudinal axis of the small intestine. ST/guanylin receptors were not observed in other strata forming the gut wall. Along the vertical axis of the human small intestine (villus/crypt unit), as well as that of most other mammals, receptor density was greatest in enterocytes located near the base of villi and in those forming the proximal portion of the intestinal glands. ST/guanylin receptors were for the most part confined to the region of the plasmalemma forming the microvillus border. In the colon of man and the other species examined, receptor density was greatest in enterocytes forming the proximal region of the intestinal glands. Receptors were present in the intestinal epithelium lining the lumen of the colon, but generally were fewer in number. The distribution of cellular cGMP accumulation responses to E. coli ST and guanylin in the opossum (Didelphis virginiana) and raccoon (Procyon lotor) revealed that proximal small intestine had greater magnitudes of cGMP responses than did the distal small intestine. Proximal colon had greater cGMP responses than distal colon, which had no significant cGMP responses to either ST or guanylin.