A form of polycystic ovary (PCO) resembling some aspects of the human PCO syndrome (PCOS) can be induced in rats by a single injection of estradiol valerate (EV). An increase in sympathetic outflow to the ovary precedes, by several weeks, the appearance of cysts, suggesting the involvement of a neurogenic component in the pathology of this ovarian dysfunction. To test the hypotheses that the change in sympathetic tone is related to an augmented production of hippocampal and/or ovarian nerve growth factor (NGF), and that this abnormally elevated production of NGF contributes to the induction of PCOS induced by EV. The animals were sacrificed after PCOS induction and the ovaries and hippocampus were sectioned and compared to the normal control. The expression of NGF was measured by immunohistochemical staining and Western blot analysis in the ovaries and hippocampus. EVinduced PCOS showed significant increase of ovarian NGF expression. Immunohistochemical expression of NGF was confined to the follicular cells and interstitial cells. Hippocampal NGF expression was not significantly changed. In conclusion EV-induced PCOS was related to the ovarian sympathetic activation which was mediated by NGF.