In brown trout surviving in acidified brook water, the plasma osmolality is reduced 15-25%. The decrease is much less than that expected from the measured reduction in the plasma concentration of Na+ and Cl-. This discrepancy cannot quantitatively be explained by the increase in plasma concentration of K+ (100%) and free amino compounds (mainly taurine; 280%) but appears mainly attributable to a reduction in plasma volume. The osmolality of heart ventricle cells is also reduced. Water content of the cells is unchanged, and there is a significant decrease in the intracellular concentration of K+, taurine and to a lesser extent glutamic acid, accounting for 25, 45 and 8%, respectively, of the osmolality reduction. These findings indicate the existence of a cell volume regulation mechanism in the fresh water brown trout which counteracts osmotic swelling of tissue cells during periods of salt loss in acidified water. The significance of this mechanism for survival under such conditions is discussed.