Monogamous social organization is characterized by selective affiliation with a partner, high levels of paternal behaviour and, in many species, intense aggression towards strangers for defence of territory, nest and mate. Although much has been written about the evolutionary causes of monogamy, little is known about the proximate mechanisms for pair bonding in monogamous mammals. The prairie vole, Microtus ochrogaster, is a monogamous, biparental rodent which exhibits long-term pair bonds characterized by selective affiliation (partner preference) and aggression. Here we describe the rapid development of both selective aggression and partner preferences following mating in the male of this species. We hypothesized that either arginine-vasopressin (AVP) or oxytocin (OT), two nine-amino-acid neuropeptides with diverse forebrain projections, could mediate the development of selective aggression and affiliation. This hypothesis was based on the following observations: (1) monogamous and polygamous voles differ specifically in the distribution of forebrain AVP and OT receptors; (2) AVP innervation in the prairie vole brain is sexually dimorphic and important for paternal behaviour; (3) central AVP pathways have been previously implicated in territorial displays and social memory; and (4) central OT pathways have been previously implicated in affiliative behaviours. We now demonstrate that central AVP is both necessary and sufficient for selective aggression and partner preference formation, two critical features of pair bonding in the monogamous prairie vole.