The conjugation of 5alpha-androstane-3alpha,17beta-diol (3alpha-A) and its 3beta epimer (3beta-A) was determined in the peripheral blood of immature female rats. About two thirds of these steroids were present in blood as sulphates and one third as glucuronides; no free steroids were detected. Administration of 3beta-A sulphate (25mug/100 g body weight/day) and of 3alpha-A sulphate (50 mug/100 g/day) from day 21 of life until the day of vaginal opening, advanced the day of the first ovulation. Administration of the 3beta-A sulphate did not induce precocious vaginal opening whereas the free alcohol was active in this respect. Implantation of 3beta-A sulphate, but not of the 3alpha epimer, into the basal medial hypothalamus resulted in the dealth of all animals within 24 h.