We evaluated the conductances for ion flow across the cellular and paracellular pathways of flounder intestine using microelectrode techniques and ion-replacement studies. Apical membrane conductance properties are dominated by the presence of Ba-sensitive K channels. An elevated mucosal solution K concentration, [K]m, depolarized the apical membrane potential (ψa) and, at [K]m less than 40 mM, the K dependence of ψa was abolished by 1-2 mM mucosal Ba. The basolateral membrane displayed Cl conductance behavior, as evidenced by depolarization of the basolateral membrane potential (ψb) with reduced serosal Cl concentrations, [Cl]s. ψb was unaffected by changes in [K]s or [Na]s. From the effect of mucosal Ba on transepithelial K selectivity, we estimated that paracellular conductance (Gp) normally accounts for 96% of transepithelial conductance (Gt). The high Gp attenuates the contribution of the cellular pathway to ψt while permitting the apical K and basolateral Cl conductances to influence the electrical potential differences across both membranes. Thus, ψa and ψb (approximately 60 mV, inside negative) lie between the equilibrium potentials for K (76 mV) and Cl (40 mV), thereby establishing driving forces for K secretion across the apical membrane and Cl absorption across the basolateral membrane. Equivalent circuit analysis suggests that apical conductance (Ga ≅ 5 mS/cm2) is sufficient to account for the observed rate of K secretion, but that basolateral conductance (Gb ≅ 1.5 mS/cm2) would account for only 50% of net Cl absorption. This, together with our failure to detect a basolateral K conductance, suggests that Cl absorption across this barrier involves KCl co-transport.