A quantitative description of the effects of sublethal concentrations of cadmium, chromium and zinc on the swimming activities of the bluegill is presented. Locomotor activities of fish in 0.1 and 0-25 mg 1-l cadmium are respectively 1.5 and 7.8 times the activities of control fish. Fish in 0 5 rng 1 -l cadmium, a concentration killing 30% in two weeks, were less active than controls. Fish in 0.05, 2.4 and 24.0 mg 1 chromium were respectively 1.2,3*6 and 6.5 times as active as controls. Fish in 0.1 and 5.0 mg 1-' zinc were respectively 1.3 and 3.8 times as active as controls. Thus metals effect hyperactive locomotor responses by the bluegill in a concentration-dependent relationship.