Branchial sodium uptake, and net proton equivalent and ammonia excretion, together with isolated filament potentials were measured in euryhaline European flounder Platichthys flesus, adapted to freshwater. External benzamil (0.05 mmol l-1), a specific inhibitor of sodium conductive channels, caused significant reductions (P < 0.05) in sodium uptake and a marked decline in net proton equivalent excretion. In contrast, 0.05 mmol l-1 external 5-(N,N-dimethyl)-amiloride, a specific inhibitor of the Na+/H+ antiport, had no effect on any of the fluxes measured. The effect of vanadate, an inhibitor of H+-ATPases, was examined at two concentrations, 0.1 and 0.5 mmol l-1, and both caused significant reductions (P < 0.05) in sodium uptake and net proton equivalent excretion. Mean resting isolated gill filament potentials in freshwater-adapted P. flesus were found to be −23.8 ± 1.1 mV (inside negative) and became less negative in the presence of amiloride, an inhibitor of sodium transport. Using microelectrodes it was possible to measure two potentials in the filament: an apical potential and a transepithelial potential. The transepithelial potential was similar to that measured in vivo, while the apical potential, may be generated by the pavement epithelial cells. In this study, it appears that sodium uptake and net proton equivalent excretion are not strictly coupled but are indirectly linked probably by a proton pump arrangement, which is thought to be a mechanism for sodium uptake in trout gills. A possible role of the apical potential is discussed.