Recent studies from our laboratory suggested that the vascular endothelium of the median eminence was involved via nitric oxide secretion in the modulation of GnRH release during the estrous cycle. To further investigate that issue, we studied the variations of endothelial nitric oxide synthase protein and mRNA in the median eminence of female rats killed at different time points of the day and/or of the estrous cycle. Endothelial nitric oxide synthase protein levels were measured by Western blot, and endothelial nitric oxide synthase mRNA analysis was performed with semiquantitative RT-PCR (for each time point, n = 4). The results revealed that endothelial nitric oxide synthase synthesis varied markedly across the estrous cycle. Indeed, endothelial nitric oxide synthase protein (n = 20) and mRNA (n = 16) levels increase significantly on 0800 h and 1600 h proestrus compared with 1400 h diestrus II. In a second step, quantification analysis were made in median eminence obtained from ovariectomized and ovariectomized, E2 benzoate primed rat. The results show a significant increase in expression of endothelial nitric oxide synthase protein as well as endothelial nitric oxide synthase mRNA in ovx-E2 primed rat median eminence. Concurrently, the levels of the cav-1 protein, a specific endogenous inhibitor of endothelial nitric oxide synthase, were measured in median eminence during estrous cycle and in ME from ovx and ovx-E2 primed rats. A significant decrease of median eminence cav-1 was noted on 1600 h proestrus and in ovx-E2 primed rats when compared with 1400 h diestrus II and ovx, respectively. Altogether, these results strongly suggest that high NO release from median eminence observed on proestrus may be due to an increase of endothelial nitric oxide synthase expression and a decrease of the cav-1 protein levels. These findings demonstrate that E2 is able to modulate endothelial nitric oxide synthase and cav-1 expression both during the estrous cycle and in experimental conditions and consequently reinforce the idea that nitric oxide acting on GnRH release, is essentially endothelial in origin. These results may also imply that variations of endothelial nitric oxide synthase expression are essential for the pulsatile/cyclic nitric oxide median eminence release observed in a previous study.