Suprachiasmatic nuclei (SCN) obtained from neonatal or embryonic 19 or 20 day rats, were grafted into the third ventricle of SCN-lesioned arrhythmic siberian chipmunks. Four out of 37 chipmunks showed reappearance of circadian rhythmicity in wheel running activity. In all 4 cases, at least one surviving graft was confirmed in the host brain. Also, vasopressin and vasoactive intestinal polypeptide (VIP)-like immunoreactive substances were found in the graft, suggesting the existence of live SCN neurons. Although the number of successful cases and the intensity of the restored rhythm was limited compared to the intra-species grafting in rats, a possibility that cross-species transplantation of SCN can restore circadian rhythmicity was shown.