Reproductive experience has significant effects on the brain, behavior and hormone profiles of the mother. Recent work has demonstrated that primiparous rats exhibit decreased dendritic arborizations in the hippocampus, and enhanced hippocampus-dependent spatial memory performance at the time of weaning compared to nulliparous and, to a lesser degree, multiparous rats. Interestingly, enhanced spatial learning and reduced dendritic arbors are seen in nulliparous female rats exposed to chronic stress or repeated corticosterone administration. Based on these observations, we hypothesized that corticosterone may be altered in primiparous rats compared to multiparous and nulliparous rats. The present study investigated whether the levels of circulating corticosterone and its binding protein, corticosteroid binding globulin (CBG), are altered with reproductive experience and pup-exposure during late pregnancy and the postpartum. Total serum corticosterone and CBG were assayed from five groups; multiparous, primiparous, nulliparous, primip-no-pups, and sensitized rats during gestation (days 14 and 19) and the postpartum period (days 1, 5, 14, 21, and 35). Results show that primiparous rats had significantly elevated total corticosterone on postpartum day 1. In addition, primiparous and multiparous rats had significantly lower CBG throughout the postpartum period than all other groups, with primiparous rats exhibiting lower levels than multiparous rats during mid-lactation. These data suggest that free corticosterone is elevated in both primiparous and multiparous dams and is elevated to a greater degree in primiparous compared to multiparous dams during lactation. Corticosterone and CBG levels were positively correlated with specific maternal behaviors during the first week postpartum in parturient rats, but not in sensitized rats, suggesting a role for corticosterone in the modulation of maternal behavior in parturient rats alone.