RZR beta is a member of the retinoid Z receptor (RZR) family of orphan receptors, and its expression is brain-specific. Recently, it was reported that the distribution of RZR beta mRNA is partially coincident with melatonin binding sites and that RZR beta is a putative nuclear receptor of melatonin. Using in situ hybridization, we investigated the developmental expression of RZR beta mRNA in the suprachiasmatic nucleus (SCN) of the rat, which is considered as circadian pacemaker. The RZR beta mRNA was first found in the dorsomedial portion of the SCN at embryonic day 20, and RZR beta mRNA expression in the dorsomedial SCN continued until postnatal day 60. This result suggests that RZR beta plays specific roles as a transcription factor in the dorsomedial SCN but not in the ventrolateral SCN during development and throughout postnatal life.