In the present study expression of estrogen receptor subtype -(ER α) and -(ER ) in the cerebral cortex, cerebellum, and olfactory bulb was investigated and compared between neonatal (1 ~ 3-days-old) and adult (250 ~ 350g) rats, using reverse transcription-polymerase chain reaction (RT-PCR). No ERa transcripts were detectable in the adult cerebellum and olfactory bulb, whereas very weak expression of ERa was present in the adult cerebral cortex. No significant difference in ERb transcripts was detectable between the neonatal and adult rats. While transcripts for both ER subtypes were co-expressed in these brain areas of neonatal rats, although ERa expression was significantly weaker than ER . Even in the cerebral cortex known to contain both ER subtypes in adult rats, ERa transcripts in neonatal rats were much higher than in adult. These observations provide evidence for the existence of different expression patterns of ER /ER transcripts in these three brain areas between the neonatal and adult rats, suggesting that each ER subtype may play a distinct role in the regulation of differentiation, development, and functions of the brain by estrogen.