The clock gene of Drosophila containing a hexamer repeat sequence (ACAGGC)n(period, per, repeat) is reported to be involved in the circadian rhythm. Young et al. (1985) cloned a mouse genomic DNA with the per repeat (cp202). In this study, temporal and spatial expres-sion of CP2.2F. and RB15 was examined in the adult rat branin by in situ hybridization. The former is a family of genes detected by using cp2.2 as a probe, and the latter is a gene corresponding to pRB15, one of the 11 rat brain cDNA recombinants homologous to cp2.2. Both genes were expressed in almost all neurons. Fluctuation of the genes under the light-dark cycle was apparently observed in the suprachiasmatic nucleus ; an intensive hybridization signal in the middle of the day and a weak in the middle of the night (p<0.01). However, the signal stayed relatively constant in other brain regions. Expression of both genes was detected in some glial and ependymal cells in the day but few at night. The present findings suggest that CP2.2F. and RB15 are involved in the circadian rhythm in the rat nervous system.