Lactating resident mice respond differently to male and female intruder conspecifics, showing defensive attack towards the former and offensive attack towards the latter. The effects of fluprazine (1-5 mg/kg) on this differential response pattern have been assessed. Although fluprazine increased the latencies of attack on male intruders, a very much more potent inhibitory effect was observed on attacks directed towards female intruders. Fluprazine also modestly reduced social investigation of female intruders and increased nest-oriented behaviour irrespective of the intruder's sex. As the pattern of attack on intruders, exploration, fear responses and maintenance behaviour all remained largely intact under drug treatment, it seems unlikely that the drug's inhibitory action on attack involves fear potentiation and/or olfactory impairment. It is suggested that the greater sensitivity of offensive attack to the inhibitory actions of fluprazine may reflect the relative degree of threat to resident parental investment posed by male and female conspecific intruders.