Stimuli animals receive from the daily light-dark cycle are the primary factors which control not only seasonal breeding, but the timing of hormonal secretion within a particular day. The overall objectives of this research proposal are to learn more about how the animal that is most frequently used in reproductive studies, i.e. the albino rat, utilizes information from the light-dark cycle to control its precise timing of ovulation, and the duration of its estrous cycle. In the first series of experiments, a proposed interaction of the pineal gland with the response of the rat to light cues will be investigated. Specifically, these experiments will determine whether the pineal gland acts to inhibit the rate at which the phase of two circadian rhythms (i.e. spontaneous running activity and ovulation) shift in response to light cues. In a second series of experiments, the role of the light cycle in determining the length of the estrous cycle will be investigated. These experiments will determine if the frequency of cyclic ovulation is regulated by light which falls throughout the daily photoperiod or by light that falls during only certain portions of the photoperiod. In a third experiment, the responsiveness of the running activity rhythm and the ovulation rhythm of the rat to light cues delivered at specific intervals throughout the entire 24 h day will be determined. With this information, a phase-response curve can be constructed for these two rhythms. This information will be useful not only to those interested in the control of the timing of ovulation in rats, but may also help to predict the responsiveness of other hormonal and behavioral rhythms to light cues.