This study analyzed the effects of deafferentation of the main, vomeronasal or both olfactory systems on the agonistic behavior of male mice in response to unfamiliar male intruder mice. In every animal, regardless of the technique used, disruption of the vomeronasal system led to consistent reductions in agonistic behavior. All animals receiving bulbectomies, vomeronasal tract transections or a combined treatment of vomeronasal cuts and ZnSO4 nasal flush reliably suppressed agonistic responding toward the intruder. Animals treated with intranasal ZnSO4 alone or those receiving control procedures continued to respond to the intruders at rates similar to pretreatment. These results indicate that the vomeronasal system is directly involved in the perception of the male chemosignals modulating agonistic behavior and, therefore, is capable of modulating the male's response to these signals.