It has been assumed that absence of testicular secretions during the period of neural differentiation results in a female neural organization, with the effect that in adulthood the animal is capable of displaying feminine behaviour. Two lines of evidence support this assumption. First, females treated with androgen during a critical period of neural differentiation fail to display female behaviour as adults (Wilson, Young & Hamilton, 1940; Phoenix, Goy, Gerall & Young, 1959). Secondly, male rats castrated during the neonatal period of neural differentiation display feminine behaviour when injected with oestrogen and progesterone in adulthood (Feder & Whalen, 1965; Grady, Phoenix & Young, 1965). The present investigation tested the possibility that testosterone propionate (TP) injected into neonatally castrated male rats during the period of neural differentiation simulates the action of endogenous testicular secretions in preventing development of the capacity for the display of feminine behaviour. The possibility that topically applied TP