Previous studies have shown a high (80-90%) rate of restoration of circadian rhythmicity in suprachiasmatic nucleus (SCN)-lesioned adult hamsters given anterior hypothalamic tissue containing the SCN taken from fetal day 13-15 donors. In the present experiments we explored the influence of age of donor on morphological and functional characteristics of the SCN graft, using tissue taken from animals at postnatal day 1, 3, 5, 7 and 10. Grafts taken from older donors tend to reach a smaller overall final size than those from younger donors, and are more likely to contain isolated, medium sized NP-positive neurons. The rate of restoration of locomotor rhythmicity following transplantation of postnatal day (PN) 1 grafts is as high as that of embryonic grafts. By PN 3, the rate of restoration falls to about 50%, and grafts of PN 7 and 10 do not restore function. As in the case of fetal grafts, there is a strong correlation between the ability of a graft to restore locomotor rhythmicity, and the presence of a cluster of vasoactive intestinal peptide (VIP) and neurophysin (NP) cells characteristic of the intact SCN within the graft. Since the period of neurogenesis for the hamster SCN occurs between day 10.5 and day 13 postfertilization, the results indicate that the SCN can be transplanted successfully well beyond the period of neurogenesis.