We have characterized the development and distribution of arginine vasopressin- and oxytocin-like immunoreactivity (AVP-IR and OT-IR, respectively) in the Brazilian opossum (Monodelphis domestica) brain. Opossum pups are born after 14-15 days of gestation in an extremely immature state before morphogenesis and organogenesis is completed, which make them an ideal model for developmental studies. Bromodeoxyuridine (BrdU) single and BrdU-AVP-OT double and/or triple label immunohistochemistry was employed to study postnatal neurogenesis of the hypothalamic paraventricular and supraoptic nuclei. Opossum pups were injected with BrdU between days 1 and 11 of postnatal life (PN) and killed at day 60 PN. Results of BrdU single label immunohistochemistry revealed that neurogenesis in the supraoptic and paraventricular nuclei was occurring into the postnatal life and completed by day 5 and 7 PN, respectively. Double and triple label immunohistochemistry demonstrated that majority of AVP and OT secreting magnocellular neurons in the supraoptic and paraventricular nuclei are formed prenatally, whereas parvicellular neurons of the paraventricular nucleus are continue to form postnatally. The general distribution of AVP-IR and OT-IR in the adult opossum brain resembled to that which had been reported for other species. In the developing opossum brain, AVP-IR was first detected at embryonic day 12 and 13 in the mesencephalon and diencephalon, respectively. In the supraoptic and paraventricular nuclei, AVP immunoreactive somata were observed on day 1 and 3 PN, whereas OT-IR in these nuclei appeared between day 3 and 5 PN. Few transiently expressing AVPimmunoreactive cells and fibers were also seen in the dorsal thalamus, forming tegmental area, and cerebellum between day 1 and 3 PN. By day 5 and 7 PN, AVP-IR and OT-IR was observed in the median eminence, posterior pituitary, and in several forebrain areas. Somata and fibers containing AVP-IR and OT-IR increased in number and amount of immunoreactivity with the increasing age. The distribution patterns of AVP-IR and OT-IR became an adult-like by day 60 PN. A sex difference in the amount of AVP-IR in the lateral septum of the opossum brain was observed at day 60 PN, which persisted into the adulthood. These findings suggested that AVP and OT play some significant roles in postnatal development of the Monodelphis.