Mean plasma cortisol concentration in juvenile chinook salmon increased from near zero to about 200 ng/ml in response to 0.5 h of severe confinement. Cortisol remained elevated for more than 6 h after release, but returned to basal levels within 12.5 h. In fish subjected to continuous severe confinement, plasma cortisol increased to about 400 ng/ml during the first 1.5 h; little further increase occurred, but by 12.5 h, mortality caused by the stress had reached 50%. In response to moderate confinement, plasma cortisol increased to different degrees in individual fish, but returned to basal levels within 6-8 days as the fish acclimated to the stress. A depression in gill Na+K ATPase was noted in juvenile salmon after they were moderately confined for about 3 wk.