Lactating female rats vigorously attack equally sized conspecific males introduced into their home cage. Under conditions of such high aggression, the previously reported pro-aggressive action of a low (5 mg/kg) dosage of chlordiazepoxide (CDP) is hardly detectable. When opponents are large, the intensity of the aggression is less than what is seen with small ones. In this situation treatment of the females with CDP increases aggression levels substantially. The importance of intruders evoking aggression was further investigated by treating different sized opponents with d-amphetamine. d-Amphetamine treatment did not lead to major changes in the defensive capacities of either types of intruder. The data demonstrate that drug effects, such as pro-aggressive actions, may be observed using larger sized opponents that are not so easily defeated and show more adequate defense than small ones. The subtleness of the dyadic interactions in maternal aggression indicates that drug effects should be considered carefully before extrapolation to other conditions.