Oxytocin and the nucleus accumbens have been extensively implicated in the regulation of maternal behavior, and the processing of pup-related stimuli relevant for this behavior. Oxytocin receptor density in the nucleus accumbens is highly variable in virgin female prairie voles, as is their behavioral response to pups, ranging from neglecting and infanticidal to full maternal behavior. We hypothesized that oxytocin receptor in the nucleus accumbens facilitates the expression of “spontaneous” maternal behavior in prairie voles. Forty sexually-naive adult females were exposed to pups for the first time and tested for maternal behavior. Oxytocin receptor binding in the nucleus accumbens and other brain regions was later determined using autoradiography. Females that showed maternal behavior (lick and groom the pups and hover over them for at least 30 s, n=24) had higher oxytocin receptor density in the nucleus accumbens (shell subregion) (P<0.05) than females that did not show maternal behavior or attacked the pups (n=16). No differences were found in other brain regions (medial preoptic area, septum, prelimbic cortex). In a second experiment, we tested whether infusions of the oxytocin receptor antagonist (d(CH₂)₅¹,Tyr(Me)²,Orn⁸)-AVT into the nucleus accumbens would block “spontaneous” maternal behavior. As a control region, oxytocin receptor antagonist was also infused into the caudate putamen. Ten females were infused bilaterally into the nucleus accumbens or caudate putamen with either 2 ng/0.5 μl of oxytocin receptor antagonist or CSF (vehicle). While five of 10 nucleus accumbens CSF-infused animals showed maternal behavior, none of the nucleus accumbens oxytocin receptor antagonist-infused subjects did (0/10; χ², P<0.01). Nucleus accumbens oxytocin receptor antagonist-infused females recovered the next day and were not different from controls. Animals infused with CSF or oxytocin receptor antagonist into the caudate putamen did not differ (four/10, four/10). This is the first study to show that the nucleus accumbens is involved in the regulation of “spontaneous” maternal behavior and that oxytocin receptor in this brain region facilitates maternal responses.