Circadian rhythms in clock gene expressions in the suprachiasmatic nucleus (SCN) of CS mice and C57BL/6J mice were measured under a daily restricted feeding schedule (RF) in continuous darkness (DD), and entrainment of the SCN circadian pacemaker to RF was examined. After 2-3 weeks under light-dark cycle with free-access to food, animals were released into DD and fed for 3 hours at a fixed time of day for 3-4 weeks. Subsequently, they were returned to have free-access to food for 2-3 weeks. In CS mice, wheel-running rhythms entrained to RF with a stable phase relationship between the activity onset and feeding time, and the rhythms started to freerun from the feeding time after the termination of RF. mPer1, mPer2 and mBMAL1 mRNA rhythms in the SCN showed a fixed phase-relationship with feeding time, indicating that the circadian pacemaker in the SCN entrained to RF. On the other hand, in C57BL/6J mice, wheel-running rhythms freeran under RF, and clock gene expression rhythms in the SCN showed a stable phase-relation not to feeding time but to the behavioral rhythms, indicating that the circadian pacemaker in the SCN did not entrain. These results indicate that the SCN circadian pacemaker of CS mice is entrainable to RF under DD, and suggest that CS mice have a circadian clock system that can be reset by a signal associated with feeding time.