The experimental measurements of passive ion permeation in rabbit gallbladder presented in this paper include: single-salt dilution potentials as a function of concentration gradient; comparison of dilution potentials for different alkali chlorides; comparison of biionic potentials for different alkali chlorides; and biionic mixture potentials as a function of cation concentration gradient. Both dilution potentials and biionic potentials yield the permeability sequence K⁺>Rb⁺>Na⁺>Li⁺>Cs⁺, a sequence consistent with simple considerations of ion-site interactions and ion hydration energies. Construction of empirical selectivity isotherms for alkali cation permeation in epithelia shows that permeability ratios are nearer one in the gallbladder and other epithelia than in most other biological membranes, indicating a relatively hydrated permeation route. Evaluation of the results of this and the preceding paper suggests the following: that cations permeate gallbladder epithelium via channels with fixed neutral sites; that the rate-controlling membrane is thick enough that microscopic electroneutrality must be obeyed; that virtually all anion conductance is in a shunt which develops with time after dissection; that apparent permeability changes with solution composition are due to the non-ideal activity factorn being less than 1.0; that effects of pH, Ca⁺⁺, and ionic strength may involve changes in the anion/cation mobility ratio owing to changes in wall charges or dipoles; and that the permeation route may reside in the tight junctions. A similar mechanism may be applicable to cation permeation in other epithelia.