The latency to onset of maternal behavior toward foster pups was examined in maternally-naive female rats treated either with uterine distention, a sham procedure, or no uterine manipulation. Uterine distention was achieved by intrauterine injection of hypertonic saline. The treatments were applied to either cycling, Day 10 pseudopregnant, or Day 11 pseudopregnant-decidualized virgins. The latency to onset of maternal behavior for both pseudopregnant groups was significantly shorter than that for the nonpseudopregnant group, when uterine distention was applied. The results suggest that uterine distention during pregnancy (during high progesterone level) may bring about both pregnancy termination (delivery) and the almost immediate maternal behavior seen at parturition, by the same hormonal mechanism.