Escherichia coli heat stable enterotoxin (ST.) caused Cl-secretion across T84 cell monolayers in a dose-dependent manner only when applied to the apical membrane surface and not when applied to the basolateral surface. Measurement of cAMP, cGMP, and free cytosolic Ca2l in response to STa suggested that cGMP alone mediated the Cl-secretory response. Studies utilizing blockers of the Na+,K+-ATPase pump, a Na+,K+,ClI cotransport system, a K+ channel, and a Cl-channel suggest that all of them participate in the Clsecretory process induced by ST.. The results suggest that the Cl-secretory response induced by STa is mediated by cGMP after the enterotoxin binds to its receptor on the apical membrane. The enterotoxin, by increasing cGMP, opens a K+ channel on the basolateral membrane as well as a Cl-channel on the apical membrane. The activation of these ion exit mechanisms, together with activations of the Na+,K+,Cl-cotransporter and the Na+,K+-ATPase pump drives Cl-exit through the Cl-channel on the apical membrane.