The heat-stabile enterotoxin produced by Escherichia coli (ST) induces diarrhea by binding to receptors on intestinal cells, activating guanylyl cyclase, and increasing cyclic GMP. High- and low-affinity receptors for this toxin have been identified previously. ST induces intestinal secretion in suckling mice in picomole doses, suggesting a role for high-affinity receptors in this process. The present studies examine the relative roles of high- and low-affinity receptors in this process. The time course of changes in free ST concentrations in suckling mouse intestine was determined after intragastric inoculation. Also, binding characteristics of high- and low-affinity receptors and their coupling to guanylyl cyclase were defined in intestinal membranes from suckling mice. Intestinal concentrations of toxin and receptor binding characteristics empirically determined were used in a dynamic model correlating fractional occupancy of high- and low-affinity receptors with intestinal secretion to estimate their relative contributions to ST-induced diarrhea.