The regulation of circadian rhythms changes with age. In humans, changes in the timing of sleep and wakefulness are especially common. In Syrian hamsters Mesocricetus auratus the free running period of the activity/rest rhythm shortens with age. The present study tested the hypothesis that critical age-related changes occur within the hypothalamic suprachiasmatic nucleus (SCN), known to contain a circadian pacemaker. Fetal SCN were transplanted into the brains of younger (20 weeks) and older (81 weeks) hamsters which had had their own SCNs ablated. The restoration of rhythmicity and the free running period of the rhythmicity were determined from continuous records of wheel-running activity. Transplantation restored rhythmicity in hosts of both ages. In older hamsters, the mean free running period after transplantation was longer than that measured before SCN ablation, but a similar lengthening of period was not observed after transplantation to younger hamsters. In addition, the mean period after transplantation was the same for both younger and older hosts even when there was a difference between the groups before SCN ablation. When the grafts were allowed to age, the mean free running period of the restored rhythms became shorter, indicating that the grafts can also undergo age-related changes. The results indicate that age-related changes specifically in the SCN are responsible for an age-related change in free running period.