A heart rate monitor device suitable for use in monitoring aerobic exercise training, automatically calculates a fitness parameter for a subject by monitoring the subject's heart rate and pacing the subject through a submaximal exercise stress test protocol. A microprocessor in the device is fed data peculiar to the individual subject, and then generates pacing signals to produce audible tones or beeps. The stride frequency, or rate of the tones varies at intervals so that the subject can exercise by running at progressively faster speeds. At the point of exhaustion, or maximum heart rate, the subject's maximal oxygen uptake capacity is calculated, based on the elapsed time, and is displayed as a fitness parameter. The subject's exercise level is automatically monitored based on his or her exercise heart rate, maximum heart rate, and rest heart rate.