Despite a large amount of data which demonstrates that the length of the day can alter neuroendocrine-gonadal activity in many animals, very little is known about the way in which information from the photic environment reaches the hypothalamic-pituitary system responsible for gonadotropin release. The pineal gland and its product, melatonin, as well as an endogenous biological clock, appear to be involved in the photoperiodic control of reproduction, but there is no unifying concept which explains the nature of this involvement. Recent experimental evidence suggests that melatonin may mediate the effects of light on neuroendocrine-gonadal activity by interacting with the circadian system. To further study the interaction between the pineal gland and the circadian system, we plan to examine the effects of melatonin on neuroendocrine-gonadal activity when the circadian system is altered by either surgical intervention or by exposing the animals to a variety of different photostimulatory and nonstimulatory light: dark cycles. In addition, we plan to examine the effect of pinealectomy and melatonin treatment on the circadian rhythm of locom tor activity, and to evaluate the effect of a melatonin antagonist on the circadian system, as well as the neuroendocrine-gonadal axis. By integrating the information obtained from the above studies, we hope to determine how the pineal gland and the circadian system are involved in the photoperiodic control of reproduction in mammals.