Two antibodies (H222 and Zymed) directed towards different sites of the estrogen receptor-α (ERα) were used for the following objectives: (1) to map the ERα in the forebrain of ovariectomized (ovx) rabbits by immunocytochemistry and (2) to determine the effect of endogenous (intact non-pregnant animals) and exogenous (ovx, estrogen-treated animals) estradiol (E2) on the population of ERα in the forebrain. Similar results were obtained with both antibodies used: dense aggregations of ERα-immunoreactive (IR) neurons were found in the infundibular nucleus (IN), the medial preoptic area (POA), the bed nucleus of the stria terminalis (BNST), and some nuclei of the amygdala. By contrast, no ERα-IR neurons were present in the ventromedial hypothalamic nucleus (VMN), but a dense aggregation of ERα-IR neurons occurred lateral to it in nucleus X. Numerous ERα-IR neurons were observed in the paraventricular hypothalamic nucleus, but not in the supraoptic or suprachiasmatic nuclei. The hippocampus proper lacked ERα-IR neurons, but the ventral subiculum in the hippocampal formation had a dense group of such cells. Intact non-pregnant rabbits showed less ERα-IR neurons in all regions tested than ovx animals. This difference was particularly clear in the medial POA, amygdala and BNST, while the IN showed only a marginal decrease. The dorsal, but much less the ventral, part of nucleus X also showed a decrease in the number of ERα-IR neurons compared with ovx animals. E2 benzoate (5 µg/day for 5 days) reduced even further the number of ERα-IR neurons in all regions except in a circumscribed area of the IN and the ventral part of nucleus X. These results show the existence of both sensitive and insensitive neurons to the down-regulatory effect of E2 on the presence of ERα. Sensitive neurons are located in the telencephalon, POA and several hypothalamic nuclei (PVN), while insensitive neurons are mainly restricted to the IN and the ventral part of nucleus X in the basal hypothalamus.