In the Syrian hamster, a sharp peak in pineal melatonin occurs toward the end of the dark period. In the present communication, we describe characteristics of this rhythm. First, the time of the initial increase in pineal melatonin is not altered by daily melatonin injections that induce gonadal regression. Second, the rapid decrease of pineal melatonin in the morning is not prevented by acutely extending the dark period. Third, a consistent nocturnal increase in pineal melatonin is not observed in a 20-h light, 4-h darkness lighting schedule. However, acute exposure to a longer dark period, at the appropriate time of day, allows expression of the pineal melatonin rhythm in these animals. Finally, the rhythm in pineal melatonin appears to be truly circadian and is tightly coupled to the circadian rhythm in running activity.