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Circadian rhythms in behavior and physiology are critical to ensure successful reproduction, but few studies have focused on how they may change with transitions in reproductive state. Those that have focused on pregnancy and lactation have revealed the emergence of new temporal patterns in a wide range of functions. The first part of this dissertation examines the hypothesis that these changes might be driven by modification of rhythms within the circadian system of the brain. I first characterized differences between non-pregnant and early pregnant laboratory rats in rhythms in protein expression of a neuronal activity marker (Fos) and in one component of the molecular oscillator (Per2) in the suprachiasmatic nucleus (SCN), which houses the primary pacemaker of the mammalian brain. In these studies I also examined the ventral subparaventricular zone (vSPZ), which is involved in modulating SCN output signals. Fos rhythms in one subregion of the SCN, the shell, and in the vSPZ underwent changes in waveform, and the peak of the Per2 rhythm in the whole SCN was phase-advanced during early pregnancy. To determine whether changes in Fos expression in the SCN shell and vSPZ were light-dependent, I next characterized differences in light-induced Fos expression in the SCN and vSPZ in non-pregnant and early pregnant rats housed in constant darkness. The rhythms of light-responsivity of the SCN shell and vSPZ were altered in early pregnancy, but in a manner that indicates that changes in these areas in animals kept in a light/dark cycle were not light-dependent. I next present evidence that rhythms in Per2 expression in some, but not all, extra-SCN oscillators within the brain also change in early pregnancy. This indicates that some aspect of early pregnancy induces modifications in the coupling of different elements of the circadian system. This could potentially cause rhythms in varying aspects of physiology and behavior to change in different ways. I also show that temporal patterns of Fos expression in some, but not all, non-circadian regulatory systems within the brain are modified in early pregnancy. Therefore, it appears that integration of concurrent changes in rhythms within and beyond the circadian system leads to a re-organization of rhythmicity within the brain. This might be part of the mechanism by which transitions from one reproductive state to another lead to the emergence of a diverse range of new rhythmic patterns in behavior and physiology. In my final experiment, I characterized rhythms in activity and temperature in females of a diurnal rodent species, the Nile grass rat (Arvicanthis niloticus), as they progressed through a series of reproductive states. In contrast to nocturnal rodents, rhythms in females of this species underwent changes in amplitude, but not in phase, during pregnancy and lactation. This suggests that diurnal and nocturnal species may need to modulate their rhythms in different ways in order to reproduce successfully.