A body-mounted measuring instrument senses the heart beat and inner body temperature of a user. The instrument includes a computing circuit and a timekeeping circuit for calculating elapsed time, total heart beats, pulse rate, and difference in body temperature from a predetermined normal temperature. A display is included for indicating various physiological parameters measured and an alarm system warns the user if an excessive heart rate and/or body temperature condition occurs. The instrument is well-suited for an athlete engaged in activities such as running, swimming and bicycling, not only as a safety device to warn the user of the occurrence of a dangerous condition, but also as a distance measuring and velocity computing device. The instrument computes distance travelled by the athlete during an exercise period based on the total number of heart beats and the increase in body temperature occurring during the exercise period. In addition to distance travelled, elapsed time, velocity, minutes per mile, aerobics points earned, calories expended and other parameters are computed on a real-time basis and selectively displayed.