The present studies concerned the location of the neural structures triggering ovulation. Using a small bayonet-shaped knife, three types of deafferentation were made and the rats were tested for ovulation. The following operations were carried out: 1. interruption of the bilateral, anterior, and superior connections to the preoptic-anterior hypothalamic area (POAHA), leaving only the posterior connections of this region intact; 2. bilateral transection in the caudal suprachiasmatic plane; and 3. unilateral arched cut in the retrochiasmatic region. Ovulation occurred in the rats, following deafferentation of the POAHA; tubal ova were found in 9 out of 13 animals, and all ovaries contained fresh corpora lutea. The vaginal smears of this group indicated nearly normal cycles in some cases, while there was a predominance of diestrus in others. Bilateral transection in the caudal suprachiasmatic plane, separating the preoptic region from the hypo-physiotrophic area, blocked ovulation in 100 % of the rats and led to development of the constant estrous syndrome. Unilateral arched cut in the retrochiasmatic area did not interfere with cyclic gonadotrophic function of the pituitary. These data suggest that the neurogenic stimulus that triggers ovulation arises, at least in part, from the POAHA. It may be assumed that this region stimulates the hypophysiotrophic area to cyclically release the luteinizing-hormone-releasing factor required for the release of the ovulating quota of luteinizing hormone. One half of the preoptic-tuberal pathway, conveying the impulses from the POAHA to the hypophysiotrophic area, appears to be sufficient for inducing ovulation.