We investigated the effects of two resin acids, isopimaric acid and dehydroabietic acid, which are both important components of wood industry effluents, on the intracellular pH of rainbow trout (Oncorhynchus mykiss) hepatocytes. In HEPES-buffered medium, at an extracellular pH of 7.7, the intracellular pH of untreated hepatocytes was 7.84 +/- 0.043 (mean +/- SEM, n = 40). The major pathway for pH regulation was calcium-activated sodium/proton exchange, as indicated by the facts that the pH decreased when sodium was removed from the medium, when the cells were incubated in the medium containing 5-(N,N-methyl isobutyl)-amiloride (MIA, a potent inhibitor of the sodium/proton exchange), and when the cells were treated with a membrane-permeant calcium ion chelator but not when the cells were treated with an inhibitor of the sodium-dependent anion exchanger. Exposure of hepatocytes to sublytic concentrations of isopimaric and dehydroabietic acid caused a pronounced decrease in intracellular pH, by 0.8 units during exposure to 50 mu M isopimaric acid and by 0.5 pH units during exposure to 50 mu M dehydroabietic acid. Resin acids decreased the intracellular pH mainly by acting as proton carriers across the cell membrane, because the decrease in pH also occurred in situations in which the activity of the sodium/proton exchange was inhibited either by removing calcium from the medium or by carrying out the experiments in sodium-free medium or in medium containing MIA.