Seasonal physiology has evolved as an adaptive strategy to changing environments with daylength (photoperiod) used as the predominant environmental cue to suit breeding and other functions to the external season. However, seasonal physiological state is determined not only by the photoperiod that is currently in effect but also by the animal's history, allowing changes in physiology in anticipation to the seasons. Many mammals and birds show internally timed, long-term (circannual) changes in seasonal physiology, synchronised to the seasons by changing photoperiods. The importance of history-dependent photoperiodic programming applies also to puberty attainment in juvenile animals, timed by the photoperiod received by the mother during gestation. In this project I investigated the effects of both types of history-dependent timing on the neuroendocrine pathways for photoperiodic regulation of seasonal physiology. In mammals, photoperiod is transmitted via the pineal hormone melatonin, which acts on the pars tuberalis (PT) to regulate thyrotropin (TSH) expression and in turn controls seasonal physiology via effects on the hypothalamic synthesis of type 2 and 3 thyroid hormone deiodinases (Dio2 and Dio3), and thus the local regulation of thyroid hormone metabolism, and downstream changes in hypothalamic neuropeptidergic signalling. Using two circannual species, the Soay sheep (Ovis aries) –a short-day breeder – and the European hamster (Cricetus cricetus) – a long-day breeder – exposed to constant photoperiodic conditions, my findings reveal that in both models, in the absence of seasonal cues, internal circannual timing is initiated at the PT control of TSH and transmitted to the regulation of hypothalamic T3 regulation and neuropeptides. Siberian hamsters (Phodopus sungorus) were placed under different photoperiods during gestation and transferred to a photoperiod of intermediate duration at weaning. Reproductive activation under these conditions was dependent upon early life exposure and this effect controls history-dependent changes in hypothalamic deiodinases. Interestingly, the gestational experience was reflected in PT TSH expression and Dio2 expression as early as birth time. The same prenatal effects were observed in a strain of seasonal mice, (Mus musculus molossinus). Overall my dissertation has established that: i) both the circannual and the melatonin signals converge on TSH expression to synchronise seasonal biological activity; ii) the photoperiodic pituitary-hypothalamic network is programmed by prenatal experience; and iii) this pathway is already functional before birth. Overall, my results highlight the PT as a conserved central site in mammals for the integration of multiple seasonal cues which via differential control of thyroid hormone levels in the hypothalamus dictates the timing in seasonal physiology.