Six independent groups of individually housed female rats of the Sprague-Dawley strain were given a 5-min aggression test on Day 0 (day of parturition), 3, 6, 9, 12, or 15 postpartum. Latency to attack male conspecifics differed reliably among groups, being shortest on Day 0. Significant group differences in frequency of aggressive acts were also found with a consistent reduction in all measures by the end of the second week of lactation. Although attack by females caused most male intruders to react defensively, 23% of intruders showed retaliatory attacks against females. The function of maternal aggression in protecting the pups from infanticide by unfamiliar males is discussed.