Scope for activity was similar for hatchery and wild rainbow trout (Salmo gairdneri), except at 25 C where values for wild trout were significantly higher than those for hatchery trout. In hatchery and wild rainbow trout, respectively, scope for activity was highest at 15 and 20 C. Active metabolism increased with temperature to 570 and 592 mg O2/kg per hr at 15 and 20 C, respectively. At 25 C, active metabolic rates of wild trout were higher than those for hatchery trout. In hatchery and wild trout, respectively standard metabolism increased from 36 and 42 mg O2/kg per hr at 5 C to 138 and 120 mg O2/kg per hr at 25 C.Scope for activity of hatchery trout was highest after 6 days starvation. Starvation had no effect on active metabolism of hatchery trout, but decreased their standard metabolism after 2 days.Scope for activity and active and standard metabolism of wild trout were similar during forenoon to those during afternoon when light simulated natural day and night and also when lighting was constant for 24 hr.Active metabolism of hatchery trout was higher during the spawning period than other periods of the year and was consistently the higher for males throughout the year.