The presence of sex differences in hippocampal morphology and function suggests that this brain region may be sensitive to the organizational actions of gonadal steroids. We therefore examined the postnatal development of estrogen receptor (ER) in the rat hippocampal formation. ER was measured by the in vitro binding of [3H]estradiol to a cytosolic preparation. Radioinert R2858 (moxestrol) was used to determine nonspecific binding. Hippocampal ER concentrations increased from birth through postnatal day (PND) 4 when levels peaked (10.05 +/- 1.2 fmol/mg protein); these were maintained through PND-7 (9.45 +/- 1.4) and declined thereafter to low levels characteristic of the adult (2.05 +/- 0.35). This ontogenic profile is similar to that found in several neocortical regions, as well as in the cingulate cortex, but is distinct from that observed in the hypothalamus, where ER levels remain high in the adult. Saturation analysis of PND-7 hippocampal cytosols demonstrated a single, high affinity binding site (Kd: 5.51 +/- 1.7 X 10(-10) M). [3H]Estradiol binding was specific in that it was displaced by radioinert R2858, diethylstilbestrol (DES), and 17 beta-estradiol but not by nonestrogenic steroids. Significantly greater ER levels were found in hippocampal nuclear extracts from DES-treated PND-7 animals compared to controls (9.74 +/- 2.27 vs. 0.49 +/- 0.24 fmol/mg DNA, P less than 0.01). The presence of functional ER was also shown by the ability of receptors to be retained on DNA cellulose. DNA cellulose column chromatography elution profiles for PND-7 hippocampal and medial basal hypothalamic (MBH) cytosols following incubation with [3H]estradiol were similar. The presence of elevated hippocampal ER levels during the perinatal critical period and evidence of functional transformation to the DNA binding state following DES treatment in vivo or estrogen incubation in vitro suggests that the hippocampus is a potential substrate for estrogen-mediated organizational events.