Golden (Syrian) hamsters are seasonal breeders. Under natural photoperiodic conditions, their reproductive systems are functional during spring and summer and atrophic during the fall and winter. This reproductive cycle can be duplicated in the laboratory by exposing the animals to artificially-created photoperiods. The endocrine correlates of photoperiod-induced changes in reproductive activity of the male hamster are fairly well characterized, but the neural control of seasonal reproductive activity has not been as extensively studied. Recent studies indicate that short day (less than 12.5 hr light/day) exposure leads to complex changes in central neurotransmitter metabolism, as well as neurotransmitter and hormonal receptor content, which, in turn, are reversed by exposure to long days or during the period of spontaneous testicular recrudescence. Many of these endocrine and neuroendocrine changes are dependent on the presence of the pineal gland, but photoperiod-induced changes in neurotransmitter metabolism have also been described in pinealectomized hamsters. Further studies of the neuroendocrine transduction of photoperiodic signals will not only provide a better understanding of seasonal reproductive and metabolic activities, but will increase our basic understanding of the neural control of the endocrine system.