Female hamsters were given localized implants of crystal-line progesterone or control (cholesterol, paraffin) substances intracranially 1.5 or 2.5 days before their next expected ovulation. The smallest-surfaced progesterone implants (1.8 × 104µm2) resulted in blockade of ovulation only when located in the medial preoptic-diagonal band region of the hypothalamus. In addition, larger-surfaced progesterone implants (2.8 × 104µm2 to 1.8 × 105µm2) also blocked ovulation in some instances when placed elsewhere in the hypothalamus, but not when placed in the hypophysis. Control substances did not block ovulation, except when structures 450 µm diameter or 710 µm in diameter were employed for placement of the implants. This blockade was unrelated to the surface area of control substance or the region implanted, but was, rather, associated with the size of the implant lesion. Testosterone implants in the medial preoptic-diagonal band region also blocked ovulation, but to a significantly lesser degree than similar-sized implants of progesterone.