Several recent studies have suggested that the neurohypophyseal peptide oxytocin may have a role within the brain to mediate various forms of affiliative behavior. As the regulation of oxytocin function may be largely determined by the number and distribution of its membrane bound receptor, we investigated oxytocin receptor distribution in two Peromyscus species selected for differences in affiliative behavior. Using in vitro receptor autoradiography with the selective oxytocin receptor ligand [125I]d(CH2)5[Tyr(Me)2,Tyr-NH9(2)]OVT ([125I]OTA), we compared Peromyscus maniculatus, a polygamous species, to Peromyscus californicus, a monogamous species. Marked species differences in the distribution of [125I]OTA were apparent in several brain areas, including olfactory pathways, bed nucleus of the stria terminalis, amygdala, dorsal lateral septum, and several cortical regions. In addition, gender differences in the binding pattern were evident in several regions, mostly due to sexually dimorphic patterns in the polygamous species, P. maniculatus. To further compare these species, the binding of a [3H]arginine-vasopressin antagonist was assessed in alternate sections from those used for [125I]OTA. Relative to oxytocin receptors, binding to arginine-vasopressin receptors showed fewer species differences, although the monogamous species appeared to have more arginine-vasopressin receptors in the neocortex and lateral septum. The striking differences in oxytocin receptor distribution are consistent with earlier studies in other rodents, suggesting that oxytocin may have an important role for mediating species-typical patterns of social affiliation.