Hypo-osmoregulatory ability in juvenile Atlantic salmon, Sulrno salur L., was improved by cortisol treatment. Implantation of a vegetable shortening pellet containing cortisol (50 mg kg−1) resulted in elevated plasma cortisol titres. Maximum cortisol levels (160–170ngml−1) were observed at days 6 and 12 after the implantation and dropped significantly by day 55. Cortisol-implanted fish in fresh water developed a twofold increase in gill Na+/K+-ATPase activity at days 6 and 12, and a threefold increase by day 55. Intestinal mucosa Na+/K+-ATPase activity was not affected by cortisol. Cortisol-implanted fish exposed to 28 ppt sea water for 48 h tended to show an improved ability to regulate their plasma osmolarity and reduce their ionic load. The osmo-regulatory ability attained at days 12 and 55 was further evaluated by exposing fish to 37 ppt sea water for 96 h. While all the control fish died relatively early in these tests, cortisol-implanted fish showed a clear reduction in their mortality rate. These results indicate that cortisol can induce biochemical and organismal changes during winter months that typify preadaptive events normally occurring in the spring.