Lesions of the suprachiasmatic nuclei (SCN) of golden hamsters prevented the testicular regression ordinarily observed in short photoperiods or after peripheral enucleation. These lesions also induced testicular regrowth in hamsters whose testes had previously been regressed by the absence of adequate photostimulation. Interruption of the primary optic tracts just caudal to the chiasm did not prevent testicular regression by short photoperiods. We hypothesize that testicular responses to photoperiod are mediated by the retinohypothalamic tract, which projects to the SCN, and that the SCN in turn control the secretion of a pineal antigonadal substance. The mechanism for this control may involve entrainment by light of hormonal rhythms in the pineal that are regulated by the SCN. Alternatives to this hypothesis are discussed.