Physiological effects of physical conditioning to water current were studied on three groups of-year-old rainbow trout,Salmo gairdneri, acclimated to 4 C. Group one (control) was raised in still water. Groups two and three were conditioned to water velocities of 20 cm/sec and 40 cm/sec, respectively, for 16 days before sampling. Muscle and plasma samples were collected before exercise and four times during subjection to 15 min of forced swimming at 53.4 cm/sec and eight times during a 24-hr recovery period. Conditioning significantly delayed the point of fatigue during forced exercise: the unconditioned fish were fatigued after about 5 min swimming, group two after about 10 min swimming, and group three at about 15 min.Physically conditioned trout showed significantly higher muscle and plasma lactate levels when fatigued, and more rapid removal of lactate from muscle and plasma during recovery from fatigue, than unconditioned trout. Exercise resulted in parallel oscillating concentration fluctuations of tissue phosphate and significant increases in concentrations of plasma phosphate in both conditioned and unconditioned fish. Plasma glucose showed no significant change during exercise but rose slightly during the recovery of all groups.