The permselectivity of the paracellular pathway of amphibian (Necturus and bullfrog) antrum was investigated with respect to intracationic selectivity and the K+ and C1- permeability ratio as a function of mucosal pH. The intracationic selectivity sequence was Rb+ > K+ > Cs+ > Na+ > Li+. Both antra showed weak cationic selectivity at pH 7.4, and at pH 4.4 for bullfrog and pH 3.0 for Necturus, the ratio PK+/PCI- was unity. At lower mucosal pH the tissues were anion selective. Treatment of the tissue with a water-soluble carbodiimide enhanced anion selectivity at higher pH; carbenoxolone, a weak acid, resulted in maintained cation selectivity at lower pH. These data sugest that carboxyl groups play a role in determining shunt selectivity; the increase in anion selectivity below pH 2.0 suggests that phosphate or sulfate groups could also be involved. © 1977, The Williams & Wilkins Co.. All rights reserved.