The anteroventral periventricular nucleus (AVPV) of the preoptic area mediates the positive feedback effects of estradiol (E₂) on LH surge in rats. Consistent with their role in female reproduction, the neurons in this region are more numerous in adult females than males. This sex difference is established due to E₂-mediated cell death in the developing male AVPV. Loss of neurons in the AVPV permanently abolishes the ability of E₂ to trigger LH surge release. However, the identity of the neurons lost during AVPV masculinization and the mechanism underlying E₂-triggered cell death have not been clearly defined to date. This dissertation shows that, developmental exposure to E₂ permanently reduces the number of dual-phenotype GABAergic/Glutamatergic (GABA/Glu) neurons, supporting the role of these neurons in female-specific LH surge release. My results identified a key role for TNFα-activated NFκB-mediated cell survival in establishing sex differences in the GABA/Glu population in the AVPV. GABAergic neurons in males had higher levels of TRAF Interacting Protein (TRIP), an inhibitor of the NFκB pathway. Thus, the male AVPV had lower levels of nuclear NFκB and its downstream target, pro-survival Bcl-2 mRNA than females. I also showed that E₂ produces these sex differences by upregulating TRIP gene expression and thus reduces the number of GABA/Glu neurons in the male AVPV. Using the N42 GABA/Glu cell line as an in vitro model for the AVPV, I verified that E₂ reversed TNFα-mediated effects on NFκB activation, Bcl-2 mRNA, and caspase activity. Moreover, E₂ could directly upregulate TRIP mRNA levels, only in the presence of TNFα. To understand the nature of this cooperativity, I cloned the proximal TRIP promoter, identified an ERE half-site and a novel TNFRE in a region 1000 base pairs upstream of the transcription start site. Mutation of either of these sites abolished the stimulatory effect of E₂ on promoter activity, suggesting that cooperative action of E₂ and TNFα is required for TRIP promoter activation. To summarize, these studies provide a novel mechanism for sexual differentiation of the AVPV in which E₂ acts cooperatively with TNFα to inhibit a neuroprotective pathway in GABA/Glu neurons of the male AVPV.