The steady-state fluxes of Na, K and Cl ions have been measured in Necturus gall-bladder epithelium by a technique that involves labelling the cells with tracer ions from the mucosal bath only, whilst the serosa is kept at low specific activity. After removing tracer, the efflux is followed into the serosal bath, revealing two exponential components. 2. The time constant of the fast component lies between 0.03 and 0.04 s-1 and corresponds to that of the extracellular space. The slow component closely matches the cellular efflux, with constants which lie between 0.14 and 0.46 X 10(-2) s-1. 3. Full unstirred-layer calculations have been performed to determine the specific activities in the mucosal solution, the cell and the corium (subepithelium). These involved measuring the diffusion coefficients of Na and Cl in the isolated corium: they are restricted by factors of 0.17 and 0.11. 4. The partial flux equations for this double-membrane system have been solved to obtain the cellular fluxes for all three ions. The results indicate that: (i) the net transcellular Na flux is 190 pmol cm-2 s-1, equivalent to the transepithelial salt flux during fluid secretion; (ii) the net transcellular K flux is effectively zero because this ion recirculates across the serosal membrane; (iii) the net transcellular Cl flux is 27 pmol cm-2 s-1, or 15% of the net transepithelial salt flux. 5. The permeability of the paracellular pathway to Cl is 1.65 X 10(-5) cm s-1 and the available driving forces will allow a maximum net electrodiffusive Cl transport of 10% through the shunt pathway. 6. 1:1 coupling of Na and Cl net fluxes at the mucosal membrane of this epithelium cannot be present, and processes other than simple electrodiffussion are required to effect net Cl transport by another route. 7. The serosal fluxes of K and Cl do not obey the flux-ratio equation. A component of these fluxes must be present which is neither active (pumped) nor passive (electrodiffusive and independent). If they are symmetrical in the steady state then the ratio of these exchange fluxes lies between 2:1 and 3:1 depending upon the size of the pump flux. They support the view that a mode of coupled K and Cl transport may be operating at the basolateral membrane of these cells.
