It was shown previously that in rats postpartum mothers are much more likely than pup sensitized virgin mothers to retrieve pups from a T-maze extension of their home cage. In the present experiment, the physiological basis of this difference was explored further by inducing maternal behavior (retrieving, crouching, licking) following hysterectomy-ovariectomy on Days 21, 16, and 10 of pregnancy. In general, the longer the duration of pregnancy, the shorter the latency to become maternal and the greater the propensity to retrieve pups from the T-maze. An additional group of Day 16 pregnancy terminated rats carrying six or fewer fetuses were not impaired with respect to onset of maternal behavior compared to their large-litter counterparts, but only a small percentage of these females retrieved in the T-maze. These findings suggest that both the late pregnancy estrogen surge and progesterone withdrawal after its peak and sustained secretion enhance maternal responsiveness and that T-maze pup retrieval (but not onset latency) may be additionally affected by the level of placental hormones.