The expression of locomotor activity by golden hamsters is temporally controlled by circadian oscillators contained within the suprachiasmatic nuclei. A genetic mutation has been found that alters the freerunning period of the locomotor activity rhythm from the wild- type value of approximately 24 to approximately 20 hr in homozygous mutants. It has been shown previously that a transplant of fetal hypothalamic tissue containing the suprachiasmatic nuclei to a host rendered arrhythmic by a complete lesion of the suprachiasmatic nuclei restores rhythmicity with the freerunning period that is normally expressed by the donor genotype. We made partial lesions of the suprachiasmatic nuclei of wild-type hosts, which did not completely abolish their circadian rhythmicity, and then placed hypothalamic implants from homozygous mutant fetal donors into the lesion site. The resulting complex patterns of locomotor activity contain rhythmic components with periods of both host and donor circadian oscillators, and suggest the presence of both stimulatory and inhibitory inputs from the circadian system to the centers controlling locomotor behavior.