Behavioral responses to stress and challenge are based in emotional and physiological arousal reactions. Adolescents with maladaptive or problematic behavior patterns, such as internalizing or externalizing problems, are likely to show atypical emotional and physiological reactions to stress. Relations between problems and reactions to stress were examined in a sample of 55 young adolescents with normal to clinical levels of internalizing and externalizing problems. Youth had their self-reported emotional states, heart rate, and blood pressure levels measured before and after engaging in two socially challenging, evaluative manipulations. Internalizing problems were associated with less positive affect but greater anxiety and cardiovascular arousal, whereas externalizing problems predicted greater hostility and positive affect but less cardiovascular arousal. The necessity of recognizing and incorporating comorbid characteristics and multiple response systems into studies of the links between problems and reactivity is emphasized in the discussion. (Contains 3 tables and 3 footnotes.)