Female Rockland-Swiss albino mice were treated for aggression during 6 successive pregnancies and lactation periods or until they ceased to re-mate. Fighting behavior was limited only to the lactation phase of each reproductive cycle. Although most animals exhibited fighting during every lactation period they completed, some exhibited fighting in some lactation periods but not in others. The intensity of aggression, as measured by the time spent fighting, was highest during the beginning of each lactation period and declined by the end of each period. Moreover, the intensity of aggression increased across the first 3 lactation periods and then declined on later lactation periods to a point below initial levels. Finally, the fighting behavior exhibited by multiparous animals was not due simply to previous fighting experience in that some multiparous mice exhibited postpartum aggression on the 6th lactation period in the absence of fighting experience during earlier lactation periods.