1. The impact of the serotoninergic receptor on the attack directed by female mice towards lactating intruders was assessed by studying the effects of 8-OH-DPAT (a serotoninergic agonist) and fluoxetine (an inhibitor of serotonin reuptake) on this paradigm at a range of doses and post-injection durations. 2. The specificity of these drug actions behaviour were examined by studying their effects on wheel running activity and performance in the open field. Non-sedative doses of 200 and 250 micrograms/kg of 8-OH-DPAT reduced attack by resident females on lactating intruders. 3. Higher doses (12-16 mg/kg) of fluoxetine reduced activity measures whereas lower non-sedative doses (up to 8 mg/kg) were without action on this aggression paradigm. 4. Additional studies with specific serotoninergic drugs are needed to clarify the role of this transmitter in attack by female mice on lactating intruders.