The effects of vomeronasal organ removal (VNX) on male mouse urine marking and aggressive behaviors were investigated. In three different stimulus conditions VNX male marking rates were about half that of sham-operated males. Aggressive behavior was tested by pairing males with male-urine-swabbed castrate males. Only 1 of the 12 VNX males displayed normal levels of fighting behavior and 6 did not initiate any fights during the aggression tests. These results indicate that normal male aggressive and urine marking behaviors are dependent on the presence of an intact vomeronasal system for their expression.