Social relationships are a source of support and comfort in our lives, as well as a source of stress and conflict. Thus, the ability to regulate responses to both positive and negative emotions and cognitions arising from social interactions can significantly influence both physical and mental health. In my talk, I will present evidence from two studies suggesting an early-emerging and persistent role of the oxytocin system in sensitivity to social support. Specifically, common variants of the oxytocin receptor gene (OXTR) predict physiological, psychological, and behavioral responses to social support and stress in both adults and infants. I will also present results from a third study exploring the role of the oxytocin system in human social approach behavior. In addition, I will discuss my ongoing research on how internal working models of relationships (e.g. perceptions and beliefs about the status and intentions of social partners) interact with neuroendocrine activity to influence responses to social stress and interpersonal conflict.