Following a period of estrogen priming, the spayeds hamster displays normal estrous behavior within 2–3 h after subcutaneous placement of progesterone. To determine the locus of the estrogen priming effect, a single midline implant of estradiol, fused within the lumen of a 27 or 23 gauge hypodermic tube, was stereotaxically implanted into various sites of the hypothalamus of females which had been spayed 9 days. At the time of lights out on the 3rd and on the 7th day following implantation, all females were injected with 200 µg of progesterone in 0.1 cm3 of propylene glycol. Females were tested for estrous behavior by placing them with experienced males for a period of 10 min prior to progesterone injection, and at 90 min, 2, 3, 4, 6 and 10 h thereafter. A 23 gauge estradiol implant provides a sufficient priming treatment if the hormone is placed in the anterior dorsal hypothalamus in an area including: the dorsal half of the anterior hypothalamic area, the ventral third of the filiform nucleus, and the area anterior to the dorsomedial nucleus; extending 0.5 mm laterally and 1.5 mm in the anterior-posterior plane and 1.0 mm in the dorsal-ventral plane. Implants containing radioactive estradiol indicated that the hormone did not diffuse to other areas of the hypothalamus. A 23 gauge estradiol tube was ineffective if implanted into other areas of the hypothalamus or if placed subcutaneously. A 27 gauge estradiol implant into the sensitive area did not constitute a sufficient priming treatment.