The spontaneous firing rate of midbrain central gray (MCG) neurons and the effect of iontophoretically applied prolactin and luteinizing hormone releasing hormone (LH-RH) on the electrical activity of these neurons was studied in untreated and estrogen-progesterone-treated ovariectomized female rats. The spontaneous firing rate of MCG neurons as well as the neuronal membrane responsiveness to iontophoretically applied prolactin was unaffected by the presence or absence of ovarian hormones. In both treated and untreated rats, the majority of neurons were not responsive to prolactin. On the other hand, the responsiveness of MCG neurons to iontophoretically applied LH-RH was influenced by hormonal treatment. A significantly larger number of MCG neurons were inhibited by LH-RH in hormone-treated animals than in untreated rats. The present work provides evidence at the electrophysiological level that ovarian hormones do not affect the firing rate of MCG neurons but do modulate the responsiveness of MCG neurons to LH-RH.