In two studies the relationship between plasma monoamine metabolites and different parameters of aggression were examined in children suffering from severe aggression and antisocial behavior. No prior studies have related measures of serotonergic function to experimentally elicited aggression and only a few included healthy comparison groups. Plasma 5-HIAA, HVA and MHPG were measured in 15 boys with a oppositional defiant disorder (ODD) and 25 normal controls (NC) (study 1), and 22 ODD and 25 NC children (study 2). On a separate occasion each subject had the opportunity to behave aggressively towards an opponent. 5-HIAA and HVA were significantly lower in the ODD than NC group and both parameters were significantly inversely correlated with aggression and delinquency. These findings were replicated in the second study. The results of the study support a role for serotonergic functioning in persistent antisocial and aggressive behavior in young children.