Arginine vasopressin modulates a number of species-typical social behaviors, including social memory in rats, scent marking and aggressive behavior in hamsters, and partner preference formation and paternal behavior in monogamous rodents. The distribution of V1a receptor binding sites in the brain varies greatly among species. Using in situ hybridization in 2 species of voles with strikingly different patterns of V1a binding sites and social behaviors, the authors demonstrate that differences in V1a receptor binding sites are due to species differences in regional V1a receptor gene expression. It is then demonstrated that the differences in receptor gene expression are associated with species differences in behavioral response to centrally administered vasopressin. Together, these data suggest that the phylogenetic plasticity of central neurohypophyseal peptide receptor expression may contribute to the evolution of species-typical social behaviors.