1. Undirectional and net fluxes of Na, Cl and mannitol were measured across the isolated short-circuited frog skin when the mucosal surface was bathed with Ringer solution at normal (7.4) or low (2.5) pH. When this solution was changed from normal to low pH, there was a marked increase in both influx and backflux of Cl and mannitol. Na backflux increased markedly but Na influx did not, resulting in a substantial decrease in net flux. 2. In open-circuit conditions at low pH both undirectional fluxes of Na increased and the potential across the skin dropped by a third. 3. The total conductance, Gt and the short-circuit current, Isc increased when the mucosal solution was changed from normal to low pH. The structural integrity of the 'tight junctions' in the epithelium was disrupted by the low pH treatment and at least 50% of the junctions examined showed a separation of the two, previously apposed, membranes. 4. It has been shown previously that when a low pH solution bathes the mucosal surface the total and shunt conductance increase; the present results demonstrate that under these conditions the short-circuit current no longer provides a good estimate of the net Na flux. We present calculations to show that protons can be the carriers for the extra charge transfer. 5. Using our values for net Na flux in open circuit and published values for the solute-linked volume flow, Jv, it could be shown that the osmolarity of the absorbate decreased at low pH.