The work and recovery pulse rates of human beings exercising at different levels of activity have been shown to be of value in judging the intensity and duration of work performed. Because of the linear correlation existing between the oxygen consumption and pulse rate, at least at the beginning of the exercise, it is suggested that pulse rate can be used as an indirect measure in calorimetric studies. The main factors involved in muscular activity fatigue are those concerned with the supply of oxygen to the muscles and the dissipation of heat produced. These two factors are dependent on the ability of the circulating system to adapt itself to body requirements. Since the heart rate is directly affected by these circulatory changes, its use as an index of fatigue resulting from muscular activity is also suggested. Submitted on April 17, 1956