As well as having widespread effects on many aspects of mammalian physiology, the hormones of both the reproductive and stress axes can directly and indirectly influence behavior. Here we review possible mechanisms through which centrally active hormones of the female reproductive system and the hypothalamo-pituitary-adrenal stress axis may interact to influence behavior and mood states during the post-partum period. We will focus primarily on the behavioral effects of selected neuropeptide hormones, in particular oxytocin, vasopressin and corticotrophin-releasing hormone. The literature documenting central behavioral effects of these neuropeptides arises almost exclusively from research in experimental animals. In particular, it has been reported that during lactation in rats there are high blood and brain levels of oxytocin. At the same time there is a reduction in corticotrophin-releasing hormone in the brain and in its secretion in response to stress. These changes may contribute to optimal maternal care of the offspring. Correlational studies of peptides and behavior in the post-partum period also support the hypothesis that neuropeptides may influence human physiology and behavior. Studies of post-partum women reveal powerful regulatory effects of lactation on the reactivity of the hypothalamo-pituitary-adrenal axis and of autonomic and immune systems, especially in the face of challenge. The integrative function of neural systems that influence both reproduction and the hypothalamo-pituitary-adrenal axis suggests one central mechanism for mediating the effects of environmental challenges.