The objective of this research was to explore the experiential determinants and the social controls of postpartum aggregation in mice. In Experiment 1, a group of seven lactating females was tested repeatedly from Day 2 to Day 22 postpartum against male mice; attacks by the females continued through Day 18. Two other groups of seven females tested only once showed diminished attacks against males by Day 14 postpartum. In Experiment 2, eight females were paired with a male for a week prior to parturition; they showed reliably fewer attacks on Day 2 postpartum than a group of nine females that were isolated prior to parturition. All of these results were obtained in a standard 3-min test. A 24-hr test on Day 4 of Experiment 2 revealed that in both groups female attacks dropped to near zero after 30 min. After female attacks declined, males initiated social behavior which occasioned vocalizations by the females. Further, males attacked in 65% of the tests and destroyed the litters of the females in 53% of the tests. The behavior of the male exerted important influences on the structure of extended female-male interactions.