The main aims of the proposed work are to specify the neural and endocrine bases for seasonal rhythms in physiology, behavior and structure of mammals. The model species to be studied is the Siberian hamster, Phodopus sungorus. A model system has been developed in which a single long day stimulates reproductive development in hamsters housed in short photoperiods. This renders feasible a new set of experiments with specific aims that include; 1) determining whether the time of weaning constitutes a special sensitive period for induction of reproductive development by light; 2) assessing whether puberty is associated with a permanent reduction in responsiveness to light; 3) establishing the roles of the pineal hormone melatonin and the suprachiasmatic nuclei in sustaining reproductive development once a long-day signal has been received by the neuroendocrine axis; 4) determining the duration of the daily photosensitive phase and how it is affected by prior photoperiodic history; 5) describing the postnatal development of responsiveness to melatonin in the neuroendocrine-gonadal axis; 6) determining the long-term behavioral consequences of a single long day given at weaning, and 7) describing the role of the circadian system and prior photoperiodic history in neurochemical mediation of seasonal cycles. Many human dysfunctions occur only at certain seasons. By specifying the role of the neuroendocrine system and day length in seasonality, we may establish principles that will permit better understanding and control of human behavior.