Lactating female rats on the 3rd to 12th day postpartum are more aggressive towards an intruder male than are nonlactating females. In this study, maternal aggressive behavior was recorded by introducing a strange male in the territory of the female and her offspring, on the fifth, seventh, and ninth day postpartum. Electrolytic lesions of the paraventricular nucleus of the hypothalamus (PVN) were performed on the fifth day postpartum. The results showed that the PVN lesion reduced the frequency and duration of attacks on the intruder. In addition, the lesion caused reduced weight gain in the pups compared to pups of the sham lesion group. The results suggest that PVN participates in the modulation of maternal aggression in rats. A possible role of oxytocin in that behavior is discussed.