Using heart rate as a primary indicator of autonomic nervous system activation, coupled with observed physical-activity data collected through 76 full shift ride-alongs, this study differentiated between physical and psycho-social stress. These data were collected as part of an earlier study of the physical requirements of police work conducted in 1998 and 1999. That study involved a systematic random sample of 297 general duty police officers drawn from all 12 municipal departments in British Columbia. The officers were surveyed through two separate questionnaires about the physical aspects of their job and about the most demanding critical incident of their prior 12 months at work. A total of 287 officers responded to the survey. One component of the study involved asking every other officer surveyed to participate in a researcher ride-along exercise. The exercise required a research assistant to observe and record all activities performed by the officers. The study found the highest physical stress to occur during pushing and pulling and fighting sequences, when the largest proportion of the musculature was active; however, police officers responding to critical incidents also manifested marked psycho-social stress and stress reactivity, being most notable during the interaction with a suspect both during the critical incident, and then during each subsequent interaction with suspects for the remainder of the shift. The mean above-resting heart rate of those involved in a critical incident remained elevated for the remainder of the shift for all tasks, including a significantly elevated heart rate during report writing in the last hour of the shift. These results show that police officers experience both physical and psycho-social stress on the job. The results also suggest that officers anticipate stress as they conduct their work, experiencing anticipatory stress at the start of each shift. Suggestions are offered for future research. 8 tables and 59 references