The central focus of this project, now in its sixth year, has been the role of the neuropeptide oxytocin in the neural mediation of affiliative behaviors. This theme has been pursued with behavioral, cellular, and chemical neuroanatomic studies. Behavioral studies, which original focused on maternal behavior, have been expanded to demonstrate oxytocin's role in female sexual behavior, rat pup responses to brief social isolation, and pair bonding in a monogamous rodent. In a non-human primate, the behavioral effects of centrally administered oxytocin's effects have continued to focus on the regulation of oxytocin receptor expression. These receptors in brain appear to be coupled to a G-protein and to be regulated mostly, although not exclusively, by heterologous influences. In receptor autoradiographic studies, we have demonstrated regional changes in oxytocin receptor expression in association with parturition and estrus, replicating earlier studies using exogenous steroid administration. The functional consequences of regional receptor differences have been pursued with both lesion and site specific injection studies. In a novel anatomic approach, we have used c-fos immunocytochemistry for the cellular localization of affiliative responses. Oxytocin cells in discrete parts of the paraventricular nucleus of the hypothalamus induce c-fos protein following specific forms of social stimulation. The accumulated behavioral, receptor, and anatomic evidence strongly supports our original hypothesis that central oxytocin projections have evolved in mammals to mediate diverse forms of attachment behavior. In comparative studies, both the steroid induction and the regional distribution of oxytocin receptors differ between closely related species selected for behavioral patterns of affiliation (e.g., monogamous vs. polygamous rodents).