We are utilizing the Brazilian short-tailed opossum, Monodelphis domestica, to study the development of the vasopressinergic system. Earlier studies demonstrated that arginine vasopressin-like immunoreactivity was present very early in the Brazilian opossum brain, suggesting a role for vasopressin in the developing central nervous system of mammals. In this study, we have utilized [3H]arginine vasopressin autoradiography to describe the distribution of arginine vasopressin binding sites in adult and developing Brazilian opossum brains. In general, arginine vasopressin binding patterns in adult opossum brains resembled those of other species. However, we found very few labelled areas in neonatal Brazilian opossum brains. At birth, only the ventral tegmental area and the nucleus of the solitary tract were labelled. Binding was not evident in the forebrain until 25 days of postnatal age. The anterior pituitary was heavily labelled from birth onward, but binding in the brain itself remained at low levels until 35 days postnatal. Heavy binding was observed in only a few areas of the brain in adults, including the dorsal part of the lateral septal nucleus, the suprachiasmatic nucleus, the dorsal and median raphe, the nucleus of the solitary tract, and the caudal part of the spinal trigeminal nucleus. Surprisingly, arginine vasopressin binding sites in the Brazilian opossum appeared much later than arginine vasopressin im-munoreactivity and, in many cases, after neurogenesis was complete. These findings suggest that the arginine vasopressin binding sites are not playing a developmental role in opossums, although the peptide is present at an early age.