The authors investigated the effects of postnatal manipulations of oxytocin (OT) on the subsequent tendency to form a partner preference in male prairie voles (Microtus ochrogaster). Neonatally, males received either an injection of OT, an oxytocin antagonist (OTA), 0.9% saline vehicle, or handling without injection. As adults, males were tested for partner preference following 1 hr of cohabitation with a nonestrous female. In a 3-hr preference test, males neonatally exposed to exogenous OT exhibited a significant partner preference, not seen in males receiving OTA or saline. Both OT and OTA voles had significantly higher levels of social contact than saline controls. A single neonatal injection of OT increased both total and selective social behaviors in male prairie voles.