The purpose of the present study was to investigate the effects of acute and chronic cocaine administration on aggressive behaviour in mice. The animals were made more aggressive by individual housing for a period of 6 weeks. Group-housed anosmic conspecifics which were not aggressive were used as intruder controls. In acute studies, cocaine induced no significant change in aggressive behaviour at low doses (0.5-5 mg/kg) but significantly decreased aggressive behaviour after doses of 10 and 20 mg/kg. Cocaine increased the isolation-induced aggressive behaviour in mice when they were injected twice daily for a week with low doses of 0.5 or 1 mg/kg. In particular, the latency to first attack was significantly decreased by the drug and the frequency of attack towards the non-aggressive intruder was dramatically increased. Higher cocaine doses (10 or 20 mg/kg) under the described treatment regimen decreased these agonistic repertories. Tolerance did not develop to the anti-aggressive effects of high doses of cocaine on continued treatment.