1. When diluted human plasma is perfused through a frog heart, a marked augmentation of the heartbeat is produced which is very similar in action to that of low concentrations of adenosine triphosphate (ATP) on the heart.2. It was established that the substance in the plasma responsible for the heart stimulation was ATP. The following tests were used: (a) the diluted plasma emitted light from firefly lantern extract characteristic of the light signal produced by a solution of ATP; (b) the stimulatory effect on the frog heart and luminescent effect upon the firefly extract were abolished by incubation of the plasma solution with the enzyme apyrase, which converts ATP to adenosine monophosphate (AMP); AMP does not stimulate the heart or cause light to be emitted from firefly extract; (c) the stimulatory substance in the plasma was eluted through a column of Sephadex G-25 in the same pattern as ATP; and (d) simultaneous assay of plasma solutions on frog heart and firefly extract produced the same quantitative result as that produced by a solution of ATP.3. The amount of ATP in plasma from the venous blood of resting subjects ranged from 0.19 to 0.95 mug/ml. (mean 0.63 mug/ml., S.D. +/- 0.25); up to half of the ATP detected could be attributed to blood platelet damage. Simultaneous arterial and venous samples of blood from four subjects at rest had mean concentrations of 0.19 mug/ml. (0.07-0.26 mug/ml.) and 0.70 mug/ml. (0.57-0.84 mug/ml.) respectively.4. The concentration of ATP in the venous effluent from exercising forearm muscles was measured. The venous concentration consistently increased over the resting values in response to exercise while in one subject little change occurred in the arterial blood concentration during the exercise. It was concluded that the ATP was added to the blood in its passage through the muscle bed.5. The origin of the ATP, including erythrocytes, blood platelets and active skeletal muscle, is discussed.