Several lines of evidence support a role for oxytocin and vasopressin in complex social behaviors, including parental care, sex behavior, and aggression. Recent studies in a monogamous mammal, the prairie vole, suggest an additional role for both peptides in the formation of pair bonds. Central administration of oxytocin facilitates and administration of an oxytocin antagonist inhibits partner preference formation in female prairie voles. Conversely, vasopressin facilitates and a V1a receptor antagonist inhibits pair bonding in males. A potential cellular basis for these effects is the species-specific pattern of expression of oxytocin and V1a receptor in reward pathways of the prairie vole brain. At a molecular level, comparative sequencing of the oxytocin and V1a receptors reveals species differences in the promoter sequences that may guide regional expression in the brain. Transgenic mice created with the 5' flanking region of the prairie vole oxytocin receptor gene demonstrate that sequencing in this region influence the pattern of expression within the brain. The unique promoter sequences of the prairie vole OTR and V1a receptor genes and the resulting species-specific pattern of regional expression provide a potential molecular mechanism for the evolution of pair bonding behaviors and a cellular basis for monogamy.