This paper examines the developmental effects of the mammalian neuropeptide, oxytocin (OT). In adults, OT is the most abundant neuropeptide in the hypothalamus and serves integrative functions, coordinating behavioral and physiological processes. For example, OT has been implicated in parturition, lactation, maternal behavior and pair bond formation. In addition, OT is capable of moderating behavioral responses to various stressors as well as the reactivity of the hypothalamic-pituitary-adrenal (HPA) axis. Neonates may be exposed to hormones of maternal origin, possibly including peptides administered to the mother in the perinatal period to hasten or delay birth and in milk; however, whether peptide hormones from the mother influence the developing infant remains to be determined. In rodents, endogenous OT is first synthesized during the early postnatal period, although its functions at this time are not well known. Experiments in neonatal prairie voles have documented the capacity of OT and OT receptor antagonists to have immediate and lifelong consequences for social behaviors, including adult pair bonding and parental behaviors, as well as the reactivity of the HPA axis; most of these effects are sexually dimorphic. Possible mechanisms for such effects, including long-lasting changes in OT and vasopressin, are summarized.