The pineal melatonin rhythm has been proposed to represent a physiological signal which mediates the effects of photoperiod in a variety of seasonal breeding mammals. Studies in other species have led to the hypothesis that an endogenous circadian mechanism controls pineal melatonin content and can adapt the melatonin rhythm to reflect shifts in the prevailing light-dark schedule. In this report, photoperiodic regulation of the pineal melatonin content was investigated in the male Djungarian hamster, a species that is a long day breeder and photo-periodic from early life. In the first study, a daily pineal melatonin rhythm persisted after 6-8 weeks of constant dark conditions in a specific phase relationship to the onset of wheel running activity. Pineal melatonin levels increased around the time of activity onset, and high values persisted for 14-17 h. In subsequent studies, adult males [previously maintained under long days (16 h of light and 8 h of darkness)] were subjected to one of three conditions: 1) a 3-h advance in the onset of darkness, 2) a delay in the time of light onset, or 3) both alterations in the photoperiod. The acute 3-h advance in onset of the dark period caused an earlier rise in pineal melatonin production by approximately 1.5 h compared to that in controls. In contrast, the falling phase of the pineal melatonin rhythm was not altered by extension of the dark period. Therefore, aspects of the pineal melatonin pattern, i.e. the rising phase and peak duration, reflected changes in the photoperiod, while the falling phase anticipated the onset of the light period predicted by the previous light-dark schedule. These observations support the hypothesis that melatonin production by the pineal is an endogenous circadian rhythm and, as a physiological signal, may convey information about the daylength.