Triads of castrated male mice (residents) were injected each day for 2 weeks with oil vehicle or 280 nmol of dehydroepiandrosterone (D). They were tested for their attack on a lactating female (intruder) introduced in their home-cage 2,24 or 48 hr after their last injection. D significantly reduced male aggressive behavior for at least 24 hr. Other groups of castrated males were injected each day with vehicle or 280 nmol of D, dehydroepiandrosterone sulfate (DS) or androstenediol (ADIOL). D, but neither DS nor ADIOL, significantly reduced their aggressive responses to intruders. Neural levels of D, DS and testosterone (T) were measured in intact males injected with vehicle or D. The concentration of D in brain was unchanged by castration and was increased about 20 fold in D injected males. The concentration of neural T was 10 nmol/g of tissue in intact males, was almost undetectable in castrated males, and was slightly increased in D injected males. Further work will be needed to identify the steroid molecule(s) responsible for this behavioral effect of D.