In order to determine the length of time neural elements must be exposed to androgen to induce sterility, 2 barbiturates, pentobarbital (PB) or phenobarbital (PhB), were injected subcutaneously into 5-day-old female rats simultaneously with 30 μg testosterone propionate (TP), or 3, 6, 9, 12 or 24 hr after TP injection. Both PB and PhB protect against the sterilizing effect of neonatal treatment with TP when injected at the same time as the hormone. When given 6 hr after TP injection, PB still protects against androgenization, but by 12 hr PB is ineffective. In contrast, the effectiveness of PhB is markedly reduced when given only 3 hr after TP injection. These data suggest that androgen can exert its inductive action on the developing brain during a very short period of 12 hr or less. They are also consistent with the working hypothesis that androgen action can be divided into 2 phases. A brief phase (approximately 3 hr in length) of androgen uptake by neurons is followed by a more prolonged phase (approximately another 9 hr) of intraneuronal biochemical differentiation. (Endocrinology82: 1010, 1968)