It is well known that castration in rats causes atrophy of the sexual organs and, to a lesser extent, of other organs. We have observed that castration causes a reduction of the liver weight due to a reduction (-10%) of the numerical density of the hepatocytes after castration (p less than 0.01). Testosterone administration at high concentration (2.5 mg/100 g b. wt.) as well as at low concentration (0.025 mg/100 g) in the castrated rats causes an increase of the liver weight due to an increase (+30%) in the cellular area (p less than 0.001) as well as in the nuclear area (+20%) (p less than 0.01) of the hepatocytes, revealing a powerful hypertrophic effect of the hormone in the liver.