Circadian timing of behavior and physiology has direct implications for both survival and reproduction. A circadian pacemaker in the hypothalamic suprachiasmatic nucleus (SCN) coordinates the daily rhythms observed in most all aspects of biology and behavior, including reproductive and immune function. However, the precise mechanisms by which the SCN influences and interacts with these systems is not entirely clear. In these experiments we tested several hypotheses relevant to unresolved questions in the complex and bidirectional interactions between the circadian, reproductive and immune systems. In Chapter 3 we utilized a novel model of circadian arrhythmia in Siberian hamsters (Phodopus sungorus) to examine the consequences of circadian disruption on reproductive neuroendocrine function and fertility. In Chapter 4 we employed this model of circadian arrhythmia to determine whether circadian disruption compromised a major aspect of a coordinated immune response (cutaneous wound healing). Finally, in Chapter 5 we examined the molecular mechanisms by which the innate immune response to infection mediates suppression of reproductive function. Together, the work confirms and extends an emerging theme that the circadian system plays a pivotal role in the coordination and mediation of normal immune and reproductive function, and identifies novel phenomenology and mechanisms by which the circadian system may accomplish these effects.