Effects on ion transport of extracellular Na+ and Cl- ([Na+]o and [Cl-]o, respectively), HCO3(-)-CO2, 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP), epinephrine, and transport inhibitors were examined in short-circuited rabbit proximal colonic mucosa. Net Na+ flux was independent of Cl- but partially (60%) dependent on HCO3(-)-CO2. Net Cl- flux was partially (70%) dependent on Na+ and totally dependent on HCO3(-)-CO2. Both fluxes peaked between 25 and 60 mM and decreased at higher concentrations. Apical Na+ influx but not Cl- influx obeyed the same pattern. The inhibition resulted from increases in mucosal but not serosal [Na+]o and not from increases in [Cl-]o. Amiloride and benzamil (0.2-0.3 mM) partially inhibited net Na+ absorption, as did 8-BrcAMP, but these effects were independent of the inhibition seen at high [Na+]o. Net Cl- absorption was inhibited by 8-BrcAMP but not by 0.2 mM amiloride. At high [Na+]o and [Cl-]o, there were a residual ion flux suggesting HCO3- secretion and, in the presence of 8-BrcAMP and amiloride or benzamil, net secretions of Na+ and Cl-, the former larger than the latter. Epinephrine, via alpha 2-receptors, reversed the ion-transport effects of high [Na+]o but did not stimulate a mucosal-to-serosal unidirectional HCO3- flux (as shown in rabbit ileum). 4-Acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (0.5 mM) and bumetanide (10 microM) had no effect on ion transport. The results suggest 1) Na+ entry via two Na(+)-H+ exchangers, one inhibited by amiloride and cAMP and the other inhibited by high [Na+]o and stimulated by epinephrine; 2) Cl- entry via Cl(-)-HCO3- exchange; 3) HCO3- secretion at high [Na+]o and [Cl-]o; and 4) cAMP-induced secretion of Na+ and Cl- at high [Na+]o and [Cl-]o.