Two doses of two very similar benzodiazepines (oxazepam 15 and 30 mg: lorazepam 1 and 2 mg) and placebo were compared 4 h post-administration on a competitive reaction time task designed to measure behavioural aggression. Forty-five subjects were assigned randomly to five independent drug groups. Subjective ratings of mood, anxiety and aggression were completed pre- and post-drug and post-task. Oxazepam and lorazepam had very similar subjective effects, but the higher dose of lorazepam increased aggressive responding on the task more than any other treatment. This may be related to the different ceiling efficacies of the two benzodiazepines.