The only direct evidence implicating the medial preoptic anterior hypothalamus (mPOAH) as an input interface through which circulating hormones influence sexual behavior is the observation than implantation of steroids into this structure will partially reinstate mating behavior in castrated rats. We plan to study this phenomenon in several ways. First, we will propose experiments designed to identify the implant procedure and the type of steroid treatment which will, when implanted into the mPOAH, both maximize the recovery of sexual behavior and minimize the diffusion of hormone to other structures. We will then utilize this procedure to explore the sensitivity of other regions which have also been shown to take up and retain steroids from the blood. Second, a radioimmunoassay procedure will be utilized to determine the extent of diffusion if implanted hormone in the brain, as well as leakage into the peripheral circulation. Third, the nature of the reinstatement produced by mPOAH steroid implants, will be studied. Fourth, a direct test will be made of the hypothesis that the reinstatement of copulatory behavior in castrates by mPOAH steroid implants is dependent on a mPOAH-medial forebrain bundle (MFB) system. If this hypothesis is correct, then the effectiveness of a mPOAH steroid implant in reinstating mating behavior should be impaired by a cut aimed at transecting mPOAH-MFB connections on the ipsilateral, but not contralateral side.