This investigation was undertaken in an attempt to develop a method of determining the condition of the heart muscle and its ability to do work. For this purpose the authors studied the length of systole as determined by the carotid sphygmogram. They found (1) that the duration of the average systole varies in different postures in accordance with the effect of gravity on the return of the venous blood to the heart; (2) that the length of the systole was not influenced by the systolic, diastolic, and pulse pressure; and (3) that it was not affected by the time of day, the time of year, age, height, weight, or the smoking of tobacco.