Hypothalamic knife cuts were employed to investigate the pathways which mediate the gonadal regression induced by short-day photoperiods in male golden hamsters (Mesocricetus auratus). Bilateral horizontal cuts placed between the suprachiasmatic nucleus and the paraventricular nucleus (PVN), as well as cuts placed through the PVN, prevented testicular regression in animals kept on a photoperiod that provided only 6 hr of light/day. Identical effects were obtained when the cuts were placed just dorsal to the PVN. Unilateral damage or sham surgery failed to block gonadal regression. The entrainment of circadian rhythms of locomotor activity to the light-dark cycle was not affected by cuts that abolished the effects of photoperiod on testicular regression. The results are discussed in the context of current models of neural control of the pineal gland and melatonin production.