Net branchial sodium uptake, and net acid and ammonia excretion, together with gill ventilation volumes were measured in euryhaline European flounder Platichthys flesus, adapted to freshwater. External amiloride or acetazolamide (0.1 mmol l-1) caused significant reductions (P < 0.05) in sodium uptake and net acid excretion, while 0.05 mmol l-1 thiocyanate had no effect on any of the fluxes measured. Ethoxzolamide (0.1 mmol l-1) however, caused significant reductions in all fluxes measured and also caused a 32% reduction in the volume of water exhaled by the gills. In this study, it appears that sodium uptake and net acid excretion are indirectly linked, probably by a proton pump arrangement which is thought to be a mechanism for sodium uptake in trout gills.