The volume of the ventromedial nucleus of the hypothalamus (VMN) of normal male rats was significantly greater than that of normal female rats. Castrationof day 1 neonatal males significantly reduced the volume of the VMN to a level comparable with that of normal females. However, the VMN volume was no longer influenced by castration on day 7. Injection of 1.25mg testosterone propionate to 5-or 15-dayold females did not have any significant effect on the volume of the VMN. These results indicate that the volume of the VMN is sexually dimorphic and is modified by internal secretion of neonatal testes.