Melatonin plasma in Swallows exhibited circadian rhythmical secretions in the LD (Light and dark, 12：12) period with and without feeding. But their average difference between at CT6 (Circadian Time) and CT18 was 3.53 ng/$ extrm{m}{ell}$ in LD period with feeding. on the other side 1.60 ng/$mell$ during without feeding. Melatonin concentration at CT6 without feeding incresed from 0.22 ng/$ extrm{m}{ell}$ to 0.93 ng/$ extrm{m}{ell}$. It is demonstrated that decresing melatonin secretion may reduce digestive function in order to ready for the migration. While the birds with feeding exhibited circadian rhythmical activity, their activity without feeding was durable. The concentrations of melatonin plasma by refeeding were 1.53 ng/$ extrm{m}{ell}$ at CT6 and 6.07 ng/$ extrm{m}{ell}$ at CT18. Melatonin plasma concentration in the night increased by more than ca. quadruple at day. This results suggest that melatonin regulates metabolism for the return to the normal metabolism condition after migration. After 3 days refeeding melatonin was secreted circadian rhythmically same as the secretions with feeding at beginning.