Subcutaneous administration of 5α–androstane– 3β,17β–diol to 21–day—old female albino rats resulted in precocious vaginal opening, precocious first estrus and ovulation, followed by regular estrous cycles. However, when the steroid was implanted into the median eminence of the hypothalamus, or injected into the 3rd ventricle of the brain, it delayed significantly the onset of puberty. The administration of the 3α epimer of this steroid was ineffective on these parameters. Implants of neither steroid into the cerebral cortex did not influence the time of normal onset of puberty. From these results it is concluded that there exist both a negative and a positive sites of action for the effect of 5α–androstane–33,17β–diol on onset of puberty. (Endocrinology92: 941, 1973)