Circadian activity rhythms that have been eliminated by lesions of the suprachiasmatic nucleus (SCN) can be restored by fetal SCN grafts. Partial lesions of the host allow simultaneous expression of both donor and host rhythms. Because partial SCN ablation produces characteristic changes in activity rhythms that are similar to those that occur with age, including shortened period, reduced amplitude, and fragmentation, we investigated the extent to which fetal SCN grafts may be expressed by an animal whose activity rhythm exhibits these age-dependent changes. The results indicate that expression of a transplanted clock is possible in an unlesioned aged host. Grafts of fetal SCN into young hosts and cortical tissue grafts into intact aged hosts have no effect. In those aged animals that received SCN grafts, three patterns of expression emerged in the subsequent locomotor activity record: complete dominance of locomotor rhythmicity by the donor; relative coordination between donor and host rhythms; and spontaneous switching between host and donor phenotypes. The results suggest that the expression of rhythmicity by the grafted SCN may depend on the relative amplitude or strength of signals produced by the host and donor SCN.