Parturient DBA/2J mice exhibit high levels of maternal aggression when confronted by an adult male Rockland-Swiss (R-S) intruder, while postpartum C57BL/6J females are passive. This strain difference is not altered by additional reproductive experience since multiparous animals continue to exhibit patterns of aggressive behavior consistent with their genotype. Reciprocally crossed hybrid females exhibit DBA-like patterns of aggressive behavior and cross-fostered females behave true to their genotype. Thus, experiential and environmental factors are not responsible for strain differences in postpartum aggression. Genotype determined differences in sensitivity to suckling-induced biochemical changes may modulate individual variation in the display of maternal aggression.