The development of aggression during the immediate postpartum period was investigated in Rockland-Swiss albino female mice. Aggression was low immediately following delivery and increased with advancing lactation. The majority of females delivered during the dark cycle, however, whether females delivered or were tested during the light or dark cycle did not influence aggression. Females ovariectomized on Gestation Day 18 displayed aggression sooner than sham-operated controls. Estrogen treatment restored typical postpartum docility in prepartum ovariectomized females. Ovariectomy also increased the number of nursing bouts, but not total nursing time, when compared to sham-operated females. These findings suggest that peripartum estrogen stimulation, directly or through influencing nursing activity, prevents aggression in females immediately postpartum.