The expression of the brain rich orphan nuclear receptor RORbeta (retinoid-related orphan receptor beta) was investigated in the mouse brain by in situ hybridization using antisense cRNA probe. Positive RORbeta mRNA signals were detected in various parts of the brain with high expression in the suprachiasmatic nucleus (SCN). In the SCN, RORbeta mRNA signals showed a peak at early daytime (ZT/CT4) and a trough at early nighttime (ZT/CT16) in both light-dark and constant dark conditions. Light exposure at subjective night did not alter the expression level. These findings suggest that RORbeta is a new member of a transcription factor possibly related to the circadian pacemaking system. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.