The Eurasian beaver (Castor fiber L.) is the largest free-living rodent in Eurasia. Beavers show seasonal patterns of reproduction (long-day breeders), stress reactions, and feeding behavior. These phenomena are associated with the function of hormonal regulatory axes: the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG). In mammals, the above processes are also controlled by orexins (OXA and OXB), neuropeptides derived from the same precursor—prepro-orexin (PPO). We hypothesized that in beavers PPO gene expression in tissues creating the HPA-HPG axes and plasma orexin concentrations differ between seasons and sexes. PPO transcript and PPO and orexin proteins were noted in the mediobasal hypothalamus (MBH), pituitary, adrenals, and gonads in April (“breeding, pregnancy”), July (“post-breeding”), and November (“pre-breeding”). In the MBH, seasonal, but not sex-dependent variations were observed in PPO gene expression; however, the transcript content increased in females in July. An interaction between season and sex was noted in the pituitary: PPO gene expression decreased in males in April. The PPO mRNA concentration in adrenals varied between seasons and sexes. Additionally, an interaction between these factors occurred and the highest PPO gene expression was noted in females in April. In the gonads, PPO mRNA levels changed dependent on season, and an interaction between season and sex was found. The PPO transcript increased in November in the testes. Seasonal variations and an interaction between season and sex were observed in plasma OXA. The highest concentration of OXA was noted in July (males) and in November (females). In beavers, PPO gene expression and plasma OXA concentration varied depending on season and these changes were modified by sex. Our results imply that orexin peptides are involved in the regulation of circannual changes of reproductive activity in beavers and seem to be associated with seasonal fluctuations in glucocorticoid secretion and stress reactions.