Lactoferrin (LF) is reported to have various bioactivity such as bone growth stimulation and analgesic activity. Recently, LF has been reported to recover the fatigue in humans after the shift-work. In this study, we examined the effect of administration of LF on the functions of the circadian clock such as the free-running rhythm as well as the light pulse-induced phase shift of the behavioral rhythm using mouse model. Since an orally-administered LF is known to be largely degraded in stomach, we used the enteric microcapsule bovine lactoferrin (eMC-LF) which protects from stomach digestion, but can be absorbed in the small intestine. The wheel-running activity of mice at 6 min-bin was automatically recorded in PC and was analyzed by CLOCK Lab. The level of mRNA of several clock-related genes was quantified by in situ hybridization method. We found that oral single administration of eMC-LF at ZT24 promoted the re-entrainment of wheel-running rhythm to 8 hr advanced LD cycle in mice. Both acute and chronic administration of eMC-LF potentiated the light pulse-induced phase shift of the wheel-running rhythm. Furthermore, eMC-LF increased the light pulse-induced expression of Per1 mRNA in the restricted area of the suprachiasmatic nucleus (SCN). We also found that eMC-LF potentiated the Per1 mRNA upregulation by i.c.v. administration of gastrin-releasing peptide (GRP), a neuropeptide involved in the photic signaling in the SCN. These results suggest that LF promotes the light entrainment of the mouse circadian clock.