A propensity for violent behaviors to develop in chronic methamphetamine (METH) abusers has been noted. The idea that increased aggressiveness might result from chronic METH administration was tested in mice after chronic (long-term intermittent, 8 weeks) or single exposures to the drug. A single injection of METH (6 mg/kg) did not augment fighting. In contrast, chronic METH administration significantly increased the number of animals that initiated bite attacks. This regimen also shortened the latency before the first attack. Latency before the first attack was shorter at 20 h after the METH injection than at 15 min after injection. Locomotor activity was not different at 20 h after METH injection, indicating that increased fighting was not secondary to METH-induced hyperactivity. METH-induced increases in fighting were not related to the duration of persistent sniffing after the initial encounter with an intruder since the duration of this behavior was significantly increased at 15 min after METH but not at 20 h post drug. These results indicate that repeated injections of METH can increase fighting behaviors and also alter social interactions in mice. Thus, intermittent administration of METH might be useful as a pharmacological model to study the biochemical and molecular bases of aggressiveness.