In winter, brook charr Salvelinus fontinalis demonstrated poor osmoregulatory ability. Saltwater (SW) transfer resulted in sharp increases in plasma osmolality and chloride levels, a decrease in white muscle water content and high mortality. A clear improvement of the hypo-osmoregulatory mechanism e.ciency was observed in spring; by May or June, fish achieved full acclimation within 1 week of SW transfer. In early fall, the time needed to restore osmotic and ionic balance increased once again. A high level of stress was detected only in winter and fall, as indicated by plasma cortisol and glucose concentrations. Great improvement in osmotic and ionic regulation was observed when brook charr were transferred to 80 C SW in February and April while impaired SW adaptability was recorded in individuals transferred to 2·50° C SW in June and August. Temperature itself was more important than a thermal gradient between fresh and salt water. Stress indicators substantiate this temperature-dependent improvement of osmotic and ionic regulation in brook charr. The results support the hypothesis of a minor contribution of cortisol to hypo-osmotic regulation in brook charr, as the cortisol response always seemed to be associated with stress in the experiments.