We examined the expression profile of catechol O-methyltransferase (COMT) mRNA and its protein in the neonatal rat hypothalamus by in situ hybridization and immunohistochemistry to clarify the sites of dopamine degradation. Strong COMT mRNA expression was observed in the suprachiasmatic nucleus (SCN) throughout its rostrocaudal extent at postnatal day 1 (P1) and P2, and the mRNA levels decreased gradually until P16. COMT mRNA was predominantly localized to the ventral and medial parts of the SCN. Intense COMT immunoreactivity was demonstrated in the ventral SCN and was detected in neuronal perikarya and processes at P1. Ependymal and microglial cells also exhibited strong COMT immunoreactivity. These results indicate that COMT may directly be involved in dopaminergic signaling in the neonatal SCN.