The interactive effects of caffeine and nicotine were studied in twelve subjects. Mood and physiologic responses to the pharmacologic components nicotine and caffeine were measured, while controlling for the sensory/behavioral aspects of smoking and coffee drinking. Two experimental sessions presented a caffeine x nicotine design, with caffeinated or decaffeinated coffee followed at thirty-minute intervals by controlled inhalations of nicotine and nonnicotine smoke. Results showed that there was a significant interactive effect of caffeine and nicotine on subjective arousal such that nicotine decreased arousal only in the presence of caffeine. These findings extend previous work showing interactive effects of caffeine and self-titrated doses of cigarette smoke in affecting subjective arousal. The effects of nicotine on subjective arousal may, therefore, depend not only on nicotine dose, but also on the presence of caffeine. Heart rate was increased by nicotine and both systolic and diastolic blood pressures were elevated by caffeine. Caffeine also potentiated the increase in diastolic blood pressure resulting from smoke inhalations, but this occurred irrespective of nicotine dose.