The critical exposure time for neonatal treatment with testosterone propionate (TP) to induce anovulatory sterility was determined. Crystalline TP was applied to a groove on the external surface of 30 gauge hypodermic tubing. These tubes, either coated with TP or with cholesterol, were implanted bilaterally in the basal hypothalamus of the 3-day-old female rat and removed after 12, 24, 48, or 72 hr. At the time of implantation carbon markers were extruded from the same tubes to indicate in the adult the site of neonatal exposure to TP. Exposure of the basal hypothalamus to TP for 48 hr was necessary before more than 50% of the treated animals were rendered anovulatory by 100 days of age. When these same animals were laparotomized at 50 days of age only 43% were anovulatory even after a 72-hr exposure to TP. Ovarian compensatory hypertrophy was not affected by neonatal intracerebral TP implantation. Although implants of TP in the region of the hypothalamus including the ventromedial-arcuate area appeared to be somewhat more effective, and after less exposure, than implants in the preoptic-anterior hypothalamic area, these differences were not significant and the present results do not support the views that either the preoptic area or the ventromedial- arcuate area represents the primary site of androgenization. The present results also appear to contrast with previous suggestions that the critical exposure to TP is 12 hr or less; possible reasons for this discrepancy are discussed. (Endocrinology94: 1161, 1974)