A series of studies was done in order to fully characterize the estrogen receptor (ER) expressed in the hippocampus of adult female rat. The structural identity among the ER mRNAs expressed in the hippocampus, hypothalamus and uterus was established by polymerase chain reaction amplification of the ER cDNA. Subsequently, the ER of the hippocampus was proved to bind DNA and β-estradiol with the same affinity as the hypothalamic receptor. Finally, it was demonstrated that systemic administration of β-estradiol determines the nuclear increase of ER levels with a time course which appears to be almost superimposable in the hippocampus and hypothalamus. On the basis of the above-mentioned evidence, it is concluded that the ER expressed in the hippocampus is structurally and functionally indistinguishable from the receptor expressed in the other hormone target tissues.