The purpose of the present investigation was to develop and test an innovative methodology to assess stress reactivity in the real world. A portable microcomputer was programmed to administer two questionnaires eight times per day for 3 days in the field. Heart rate was measured simultaneously using a reliable and valid ambulatory monitor (Sport Tester PE-3000). Data were analyzed in terms of stress versus no stress, based on subjects' perceptions of the 24 events. It was found that heart rate was significantly higher in the stress condition and that physical stressors contributed significantly to heart rate variability, as did cigarette smoking. Three emotional responses were evident when subjects were stressed: anxiety, hostility and depression; the former two emotions also contributed to heart rate variability. The heart rate data were compared to a number of other studies conducted in the field and found to be strikingly similar. It was concluded that the new technologies currently available allow for the measurement of stress reactivity in the field.