The magnitude of cellular and shunt conductance of Necturus gastric antral mucosa was studied by (a) comparing the cellular PD response to transepithelial PD response during changes of ionic activity in the serosal bathing solution and (b) by measurement of current spread within the epithelial sheet. Using constant product KCI changes cellular resistance was 6,788 Ωcm2 and shunt resistance was 1,803 Ωcm2. Deletion of HCO3− from the serosal solution produced similar but quantitatively smaller changes in PD. Using HCO3− deletion cellular resistance was 7,338 Ωcm2 and shunt resistance was 1,973 Ωcm2. Measurement of current spread within the mucosa avoids changing ionic gradients yet gave very similar results; cellular resistance was 8,967 Ωcm2 and shunt resistance was 2,947 Ωcm2. The shunt contribution to transepithelial conductance ranged from 75.2 to 79.0%. Shunt selectivity was assessed using KCl dilution potentials, where mucosal dilution gave a small change in tissue PD compatible with an anion/cation selectivity ratio of 1.16 across the shunt, whereas serosal dilution effect was dominated by a PD change across the serosal membrane of the cell. © 1975, Rockefeller University Press., All rights reserved.