The means by which diazepam alters the social behavior of male LAC mice was investigated by analyzing 6-rain dyadic social encounters between untreated, individually housed resident males and experimentally manipulated, group-housed intruders. Experiment 1 showed that at 24h and particularly 14days after access to 0.125 mM diazepam solution, drugged intruders were attacked more when placed into a resident's home cage than were intruders receiving vehicle. After 24h, but not 14days of treatment, drugged intruders performed fewer elements of static flight. However, on both occasions they showed proportionally less flight behavior relative to the amount of aggression residents directed towards them. In experiment2, intruders marked with mouse urine taken from donors which had ingested the 0.125 mM diazepam solution for 24 h were attacked considerably more by residents than were intruders marked with water or normal mouse urine. Urine samples taken after 14days of drug treatment evoked additional increases in sexual and investigatory elements in unmarked residents. The results show that, in pairs of mice, the rise in aggression associated with sustained diazepam treatment, unlike the changes in flight, arises indirectly and probably through a drug-induced change in the olfactory properties of mouse urine.