Data are presented which demonstrate acclimation of brown trout (Salmo trutta) to an acid medium. Sodium fluxes, transepithelial gill potentials and plasma electrolyte concentrations were measured in brown trout exposed to an acid medium of pH 6·0 for a period of 6 weeks.

Sodium loss rates in acid exposed fish are reduced compared to normal fish, and sodium uptake is insensitive to external pH following long-term acid exposure.

Changes in gill potential and sodium turnover rate following acid exposure can be accounted for by changes in the relative permeability of the gills to sodium and chloride. In neutral media the permeability ratio φNa+/φC1 = 1·12 but this falls to 0·76 during acid exposure.

The ability to acclimate to acid conditions seems to depend largely on changes in gill permeability to sodium.

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