Abstract. The temporal changes in excitability in ventromedial hypothalamic neurons which project to the midbrain central gray caused by chronic estrogen exposure were measured in ovariectomized rats. In 66 urethane-anesthetized animals which had either no capsule, or had a capsule containing estrogen implanted for 24 h, 48 h, 72 h or 96 h, 189 antidromic action potentials were recorded in the ventromedial hypothalamus (VMN) in response to stimulation of the midbrain central gray (CG). The antidromic activation threshold was lower in 40 recordings made from the VMN in the 72-h or 96-h group than in 67 from the no-treatment, 24-h or 48-h group. The decrease in the threshold coincided with the emergence of the lordosis reflex. In a separate cohort of ovariectomized rats, any absence of estrogen for 2-48 h before and during the measuring abolished the neuronal as well as the behavioral effects at 72 h. The results suggest that the lordosis reflex induced by estrogen alone depends on the excitation of VMN efferents to the CG. In addition, the indispensability of estrogen at the time of electrophysiological and behavioral evaluation indicates that a certain direct neural action of estrogen is involved in the behavioral activation. Implants of minute amounts of crystalline estradiol showed that the ventromedial nucleus of the hypothalamus (VMN) is the principal site for estrogen action to facilitate the lordosis reflex [9]. Stimulation and lesion studies have shown that the VMN facilitates the lordosis reflex via its descending projection to the midbrain central gray (CG) [10][11][12]. Estrogen-induced excitation of VMN neurons with axons to the CG in ovariectomized rats [13][14][15] corroborates the behavioral results. Experiments on neonatally gonadectomized males and neonatally androgenized females, in which the neural effects of estrogen in parallel with its behavioral capability to induce the lordosis reflex [16], provide additional evidence for the association of the neural effect of estrogen with facilitation of the lordosis reflex.In the present study we attempted to establish a