Female rats were divided into 3 groups ; ovariectomizcd(OVX), low dose cstrogen-treated, and high dose estrogen-treatcd fats. NA DPI-1-diaphorasc enzyme histochemistry and nitric oxide synthase(NOS) immunoeytochemistry were prelormed using free floating Sections. The number of positive cells werc counted in the anieroventral preoptic nucleus (A VPv) and medial preoptic nucleus (MPN) of the hypothalamus. The number of NADPH-d and NOS-positive cells were not influcnced in the AVPv but incrcased in the MPN of high dose estrogen-treated rats. NOS mRNA in situ hybridization showed no significant difference in the number of positive cells and the density of silver grains in the positive cells between OVX and low dose estrogen-treated rats. The microdialysis study revealed no significant difference in the production of NO in the AVPv between OVX and low dose estrogen-treated rats. These data may suggest the possibility that the NO production in the rostral preoptic area is not affected by low dose of estrogen but by high dose of estrogen. In addition, low dose estrogen enhanced the NMDA evoked GABA release in the AVPv in the microdialysis study. This result suggested the relation of NMDA receptor in the GABA release.