To see if the vasopressinergic system, implicated in maternal behavior, changes during parenthood in different directions in males and females, I examined the pattern of vasopressin immunoreactivity in prairie voles in which both mother and father care for young and meadow voles in which only the mother cares for young. For both species, I compared the densities of vasopressin immunoreactive (AVP-ir) fibers in males and females that were either sexually naive or in parental state. In both species, AVP-ir fiber densities in the lateral septum (LS), and the lateral habenular nucleus (LHN) were higher in males than in females regardless of their parental state. AVP-ir fiber density in the LS and the LHN was reduced in parental prairie-vole males, while no differences were found in females. The AVP-ir in the LS of parental meadow-vole males and females did not show changes, while the fiber density in their LHN was increased. To see if AVP immunoreactivity in male prairie voles changes before or after the birth of pups, I compared AVP-ir fiber density among males and females that were either housed separately or housed together until after mating (3-P), during early gestation (13-P) or late gestation (21-P), or after the birth of pups (26-P). Under all these conditions, AVP-ir fibers in the LS and the LHN were much denser in males than in females. In males, the lowest density of AVP-ir in the LS and the LHN was found in 3-P males. An intermediate density was found in 13-P and 26-P males. An equally high density was found in sexually naive (0-M) and in 21-P males. In females, no changes in AVP immunoreactivity were found. The decrease in AVP-ir fibers of 3-P males occurred only in presence of a female. Changes in AVP immunoreactivity of male prairie voles could be related either to sexual activity which takes place after the first 3 days of cohabitation and again around pups' birth, behavioral changes associated with pair bonding, or induction of paternal behavior.