Removal of the vomeronasal organ (VNX) from female mice had little effect upon fecundity, nest building and pup retrieval. Surprisingly, VNX significantly affected aggression by lactating females. Normally, lactating females will attack and fight intruders placed into the home cage, especially if the intruder is an unfamiliar male. In Experiment 1, we determined that VNX prior to sexual experience totally eliminated this type of aggression. Intact females were highly aggressive, but much more so to unfamiliar than to familiar males. Copulation prior to surgery did not ameliorate the effects of VNX; again, none of the VNX females were aggressive (Experiment 2). In Experiment 3, attacks and fights by previously aggressive primiparous females also were eliminated or significantly suppressed following VNX. Even when the testing occurred later during the postpartum period and for extended periods of time, this same suppression of aggression held following VNX. We conclude that aggression by lactating female mice is under strict mediation by chemosignals detected by the vomeronasal organ.