Respiratory sinus arrhythmia (RSA), an index of parasympathetic-linked cardiac activity, is a reliable biomarker of emotion regulation capabilities, given appropriate stimulus conditions. Low resting RSA and excessive RSA reactivity (withdrawal) to emotion evocation are associated consistently with clinical levels of internalizing and externalizing psychopathology. Moreover, concurrently elevated levels of both internalizing and externalizing symptoms predict greater RSA reactivity to emotion evocation than either alone. No studies, however, have examined whether internalizing symptoms moderate longitudinal associations between RSA and externalizing psychopathology among preschoolers treated for behavior problems. I assessed resting RSA and RSA reactivity to emotion evocation, both before and after preschool children (n=99) with ADHD participated in the Incredible Years intervention, and compared their responding to that of age-matched, typically developing controls (n=33). Different associations between externalizing symptoms and RSA were observed depending on levels of internalizing symptoms. Children with high externalizing scores and low internalizing scores exhibited reductions in resting RSA from pre- to post-treatment, whereas children with high externalizing scores and high internalizing scores exhibited increases in resting RSA. Finally, high externalizing scores and high internalizing scores predicted greater RSA withdrawal to emotion evocation following the intervention. Implications for contemporary models of emotion regulation are discussed.