Prenatal androgens masculinize postnatal reproductive neuroendocrine function and behavior in sheep. Testosterone treatment of pregnant ewes during midgestation masculinizes sexual behavior and luteinizing hormone secretion in female lambs, presumably in part via aromatization and estrogen receptor (ESR) binding in the brain. We hypothesized that male and female sheep also differ in the number and distribution of ESR-containing neurons. If so, ESR expression should be sensitive to prenatal hormones delivered exogenously or in situ. ESR alpha (ESR1) was compared by immunocytochemistry in male and female lambs at the end of gestation, as well as in fetal females exposed prenatally to testosterone or dihydrotestosterone. ESR1-positive neurons were abundant in the posteromedial bed nucleus of the stria terminalis (BSTpm), medial preoptic area (MPOA), posterior medial amygdaloid nucleus (MeP), amygdalohippocampal area (AHi), ventromedial hypothalamic nuclei (VMH), and arcuate hypothalamic nuclei (ARC). In females, the ARC had the largest number of stained cells (mean ± SEM, 475.6 ± 57.4 cells/0.173 mm²), while staining intensity was greatest in the MPOA (mean ± SEM gray level, 31.3 ± 5.3). The mean ± SEM integrated gray level (IGL) was high in the ARC (0.63 ± 0.13) and in the MPOA (0.51 ± 0.08). The mean ± SEM IGL was low in the MeP (0.31 ± 0.10) and in the BSTpm (0.21 ± 0.06), while it was intermediate in the AHi (0.36 ± 0.10) and in the VMH (0.37 ± 0.07). ESR immunostaining was not significantly different in male and female fetal lambs, nor in females fetuses exposed prenatally to androgens (P > 0.05). However, ESR1 staining was significantly increased in the ARC, MPOA, and AHi of adult rams vs. adult ewes. These results suggest that brain ESR immunoreactivity in fetal lambs is unlikely to account for postnatal sex differences in reproductive function. Instead, sex differences in ESR emerge postnatally.