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Attachment may be described as an adaptive capacity to regulate emotions under stressful conditions, through voluntary and selective social engagement behaviors. Attachment researchers have produced evidence for this in children of all ages. We wondered whether attachment representations later in life could mediate stress responses, in the absence of familiar partners. This is an important issue since disorganization of stress responses has been evidenced in a number of mental illnesses. One hundred adults were submitted the Adult Attachment Interview (AAI) and the Trier Social Stress Test (TSST). During the TSST, blood was collected at different points and sampled for various hormones. We were firstly able to confirm that, as animal studies suggest, peripheral Oxytocin (OT) is secreted by in humans when under stress. Therefore, OT can be considered as part of the stress response system. Secondly, we demonstrated that peripheral OT secretion is associated with attachment representations, and that secure individuals produced higher levels of OT under stress as compared with insecure ones. Many studies in humans have shown that social intimacy produces internal secretion of OT, and that external OT infusion downregulates the stress system. As a consequence, our results suggest that human secure adults may be able to downregulate their stress system even in the absence of social intimacy, whereas human children need social intimacy in order to do so. These data also help illustrate the neurobiological bases of the interaction between the attachment and stress systems and confirm, at the neuroendocrinal level, the basic hypothesis of John Bowlby’s theory—that attachment and stress are closely associated from both a clinical and an evolutionary point of view.