Depending on the moment of implantation, intra-cerebrally-placed progesterone pellets either induce or inhibit ovulation specifically if they are placed into the ventromedial nuclei of the hypothalamus. Similar implants placed in the pre-optic area, the amygdala, or the hippocampus are ineffective. Implants in the ventromedial nuclei can lengthen the ovulatory cycle by only one day. A hypothesis is presented to explain the effect of progesterone on the secretion of gonadotropins in different experimental circumstances.