Testosterone, and to a lesser degree methandrostenolone, was shown to influence cardiac growth in immature male rats by affecting protein synthesis and degradation. The nature of cardiac responses to androgen appear to depend on the prevailing experimental conditions. Protein synthesis was inhibited in the castrate rat and was stimulated by subsequent treatment with androgen. Under conditions of induced overgrowth of the ventricles, androgens gave rise to an attenuation of the effects of aortic constriction on ventricular mass and blood pressure involving smaller changes in protein synthesis and proteolysis. Concentrations of testosterone receptors in ventricular cytosol further indicated that the myocardium is more sensitive to androgen action during the prepubertal phase of the life-span. Changes in amount and properties of the receptors showed them to be functional and responsive to castration, aortic constriction, and administration of the androgens. The androgens affected cardiac protein balance by stimulating the incorporation of radiolabelled amino acid into protein in vivo. They also appeared to influence proteolytic processes involving lysosomal hydrolase activities, but their actions were either stimulatory or inhibitory depending on the internal environment. The heart is a target organ for several hormones including androgen, and our findings fortify the notion that hormone action needs to be investigated alone and in association with other endocrines.