We investigated the impact of acute tryptophan depletion (ATD) and reduced brain serotonin synthesis on physiological arousal in 15 young people with ADHD participating in an aggression-inducing game. ATD was not associated with altered physiological arousal, as indexed by electrodermal activity (EDA). Baseline aggression was negatively correlated with the mean ATD effect on EDA. In accordance with the low arousal theory related to aggressive behavior, subjects with reduced physiological responsiveness/lower electrodermal reactivity to ATD tended to display elevated externalizing behavior.