Cellular growth responses of rainbow trout ( Salmo gairdneri ) fed different levels of dietary protein (35, 45, and 55%), with and without the anabolic steroid ethylestrenol, have been studied over a 60-day period. With an increase in dietary protein, total liver proteins increased in fish fed the steroid-free (control) diets, whereas no change occurred in the other tissues. Muscle and spleen RNA was unchanged, but RNA increased in liver, kidney, and brain. The DNA content increased in muscle, decreased in brain, but remained constant in liver, kidney, and spleen. Feeding the ethylestrenol-supplemented (experimental) diets resulted in an increase in total proteins, RNA, and DNA of kidney over the respective control value at each level of dietary protein. In the other tissues, total proteins and DNA were essentially unchanged, but total RNA content decreased in liver and increased in muscle in the experimental groups. It is concluded that in trout, the dietary protein level exerts marked differential effects on cellular growth parameters ( RNA DNA , RNA/protein, protein/DNA), which are further modified by steroid treatment. Evidence that cellular growth responses in muscle keep pace with total body growth was also indicated.