The value of the denervated heart as an indicator of biological reactions in the surviving animal is pointed out. It is a representative organ, existing in the fluid matrix of the body, performing its normal functions, with activity capable of being easily recorded, and influenced only by changes in the blood which perfuses it. The hitherto recognized nerves supplying the cat''s heart are described. When these are severed and all humoral agents known to affect the heart rate are excluded from action, the heart, nevertheless, is capable of markedly accelerating if the animal changes from a resting to an active state. Evidence is presented showing that accessory fibers from upper thoracic ganglia below the stellates mediate the faster beat. When these are removed the heart rate remains remarkably steady within approximately 12 beats in spite of vigorous activity. Details of the procedure for completely denervating the cat''s heart are described.