Evidence exists for the localization of the newly identified estrogen receptor  (ER) within the rat paraventricular nucleus (PVN) and supraoptic nucleus (SON), regions which lack ER␣. Presently, we investigate whether ER-like-immunoreactivity (-ir) is found within cells of several major neuropeptide systems of these regions. Young adult Sprague-Dawley rats were ovariectomized (OVX), and 1 week later half of the animals received estradiol-17 (E). Dual-label immunocytochemistry was performed on adjacent sections by using an ER antibody, followed by an antibody to either oxytocin (OT), arginine-vasopressin (AVP), or corticotropin releasing hormone. Nuclear ER-ir was identified within SON and retrochiasmatic SON, and in specific PVN subnuclei: medial parvicellular part, ventral and dorsal zones, dorsal and lateral parvicellular parts, and in the posterior magnocellular part, medial and lateral zones. However, the ER-ir within magnocellular areas was noticeably less intense. OT-͞ER-ir colocalization was confirmed in neurons of the parvicellular subnuclei, in both OVX and OVX؉E brains (Ϸ50% of OT and 25% of ER-labeled cells between bregma ؊1.78 and ؊2.00). In contrast, few PVN parvicellular neurons contained both AVP-and ER-ir. As well, very little overlap was observed in the distribution of cells containing corticotropin releasing hormone-or ER-ir. In the SON, most nuclear ER-ir colocalized with AVP-ir, whereas few OT-͞ ER-ir dual-labeled cells were observed. These findings suggest that estrogen can directly modulate specific OT and AVP systems through an ER-mediated mechanism, in a tissuespecific manner.The newly identified estrogen receptor  (ER) has been shown to exist in rat (1), human (2), and mouse (3), but the physiological role(s) of this receptor remain(s) unknown. The ligand binding characteristics have been found to be generally similar to those of the ''original'' ER, now known as ER␣, despite only a 55-60% homology in the C-terminal ligand binding domain of the two receptors (1-4). Likewise, ER appears capable of activating the expression of an estrogen response element-containing reporter gene construct, in a hormone-dependent manner, at very low hormone concentrations (1-3). This finding is not surprising, considering that the DNA binding domains of the two isoforms are 95-97% homologous. However, the remaining domains of these two steroid receptors show no homology, and recent evidence suggests that ER␣ and ER possess distinct transactivation functions (3, 5).Another clue for a distinct role for this ''second'' ER may be its anatomical distribution. Although there appears to be some overlap, the distribution of the two receptor subtypes appear to be quite different, based upon recent descriptions of ER transcript localization in the rat (4). In the rat brain specifically, the paraventricular nucleus (PVN) and the supraoptic nucleus (SON) have been identified as having large concentrations of cells containing ER mRNA (6, 7) or immunoreactivity (8). Although [ 3 H]estrogen-co...