The conductance, ga, and electromotive force, E, of active ion transport across toad bladders mounted as sacs were estimated from electrical measurements made before and after addition of sufficient ouabain (1.89 X 10(-3) M) to eliminate spontaneous potential. The ratio of net sodium transport (estimated from bidirectional fluxes) to external current in bladders voltage clamped to 0 mV was significantly less than unity in a normal medium containing HCO3- and Cl- ions, and also when Cl- was replaced with SO42-. However, when acetazolamide was added or when HCO3- was replaced by phosphate, short-circuit current and net sodium transport became equal. Spontaneous potential, E, and ga were all reduced about 20% by these maneuvers. The response of bidirectional sodium fluxes to voltage clamping at 0 mV or 120 mV in a bicarbonate-free medium was otherwise similar to that observed in a normal medium: net flux varied linearly with potential and calculated fluxes in the active transport path indicated a value significantly greater than unity for the empirical constant Q in the equation for change in the flux ratio, f, with change in potential, psi, viz., delta ln f = Q(ZF/RT)delta psi, similar to the high value for this constant that we have found in a bicarbonate-containing medium. We conclude that bicarbonate ions facilitate active sodium transport and also may be actively transported from serosa to mucosa in Dominican toads. However, coupling between bicarbonate and sodium fluxes does not account for the high value for Q for sodium in the active transport path.