In the Djungarian hamster, short photoperiods induce regression of testes and accessory glands. There is evidence that the length of time melatonin levels are elevated is the signal conveying the photoperiodic effects to the neuroendocrine axis. When the temporal course of decompression of the pattern of pineal melatonin content was followed after a change from long to short photoperiods by symmetrical extension of the dark time, extension proceeded first into the morning hours. The present study investigated whether, after transition from long to short photoperiods by either extending the dark period into the morning hours, or starting the darktime at noon, differences in the gonadal reaction and in the rate of extension of the pineal pattern ensue. Three and 7 weeks after transition from long to short photoperiods highly significant differences were found in the rate of involution of testes and accessory glands. Regression was more advanced after extension of the dark period into the morning hours. The length of time that pineal melatonin content was elevated also differed markedly between the two groups. After 3 weeks in short photoperiods, the length of the melatonin peak was more than 9 h when the dark period was extended into the morning, but only about 7 h if it was extended into the afternoon. It is suggested that the different rate of gonadal regression after different ways of transition into the same photoperiod is due to the different rate of decompression of the melatonin pattern.