In the present study, we have examined the adrenergic responses of Arctic charr (Salvelinus alpinus) erythrocytes acclimated to different temperatures (2, 8 and 14 degrees C) during different seasons. We measured the changes in cellular water and ion contents after noradrenaline stimulation using different noradrenaline concentrations and external pH values. Furthermore, the effects of acute temperature changes on the magnitude of the adrenergic response were studied. The adrenergic response of Arctic charr erythrocytes showed pronounced seasonal variation. The [Na+]/[Cl-] accumulation ratio after adrenergic stimulation was greatest in May, indicating an enhanced activity of the Na+/H+ exchanger. The noradrenaline-induced change in [Na+](i) was greatest in spring. In addition to a seasonal effect, the exchanger seemed to be most active in erythrocytes from charr acclimated to low temperature (2 degrees C) early in May: the EC50 value was lower and the calculated maximal increase in [Na+](i) was greater in the 2 degrees C-acclimated group than in the other acclimation groups. In contrast, acclimation to different temperatures did not affect these responses (measured at a constant temperature) in February. An acute temperature change has a smaller effect on the adrenergic response of Arctic charr erythrocytes than on rainbow trout (Oncorhynchus mykiss) erythrocytes.