Bilateral peripeduncular (PPN) lesions made on the seventh postpartum day (L7) with either radiofrequency (RF) current or N-methyl-d,l-aspartic acid (NMDA)/phosphate buffered saline (PBS) reduced maternal aggression (MA) and partially inhibited lactation without producing significant deficits in other items of maternal behavior (MB). RF-PPN lesions did not interfere with prolactin secretion, which suggests that there was deficient oxytocinergic activity. The deficit in MA was not due to interruption of afferent suckling input to the PPN: either thelectomizing females (day L6) or producing bilateral knifecuts in the mesencephalon (placed caudal to the level of the PPN; Day L7) had no effect on MA, but both procedures impaired lactation. Deficits in MA produced by RF-PPN lesions developed gradually between Days L4 and L7; lesions made either prepartum or on Day L1 did not impair MA or MB. Deficits in lactation first appeared after RF-PPN lesions on Day L1.