Chum and coho salmon fry, when immersed in solutions of methyl testosterone, synthetic thyroxine sodium, or the antithyroid drug, thiourea, do not show any new or different patterns of behavior. Chum salmon fry do, however, show quantitative changes in the rate of swimming and in the intensity of schooling. Less marked variations are seen in aggregating behavior of the treated coho. Control chum fry swim in the brightest part of a horizontal light gradient while coho fry, coho smolt, and trout prefer darker areas. Minor changes in this photoresponse, observed in fish treated with drugs, are probably produced by variations in the amount of swimming.