The investigations described in this paper were undertaken with a view to obtaining further information concerning dyspnea in patients with heart disease. Attention has been directed particularly to the moderate degrees of dyspnea which are incidental to the life of most persons in whom cardiac disease is a limiting, but not an incapacitating lesion, and certain aspects of the respiration and circulation have been studied while the subjects were performing exercises which did not exceed in amount or differ in kind from what they were accustomed to in normal life. The problem has been to determine, so far as possible, the differences in reaction to exercise which account for the fact that patients with heart disease become short of breath as the result of an amount of exercise which does not affect normal persons. It seemed to be important to avoid the complicating factor of muscular fatigue by selecting a