Heart rate reactivity to a standardized laboratory stressor (mental arithmetic under timed and competitive conditions) was measured in 37 men and women, drawn from white- and blue-collar occupations. In addition, heart rate reactivity data during 4 hr of work were gathered on each of these subjects and analyzed to determine (1) the presence during work time of reactivity equal to or greater than that to the mental arithmetic stressor; (2) the incidence of reactivity of 5, 10, 15, and 20 bpm during work time, and (3) the predictability of frequency of work time reactivity by mental arithmetic reactivity. Data suggested that, although there were several incidences of heart rate reactivity responses during work which were similar to those noted in the laboratory, these work heart rate responses were not able to be significantly predicted by laboratory heart rate responses.