The heat of the reaction ATP4 + H2O [forward arrow]ADP3- + HPO42- + H- was measured at pH 8.0 with myosin as enzyme. After subtracting, according to the buffer system, the very different heats of neutralization, which we have measured separately, there remains for the cleavage of the "energy-rich phosphate bond" [DELTA] H = - 4800 cal. Conclusions from this result, compared with the classical assumption of about - 12,000 cal., are discussed briefly. The principle and performance of the microcalori-metric method here used for the first time is described.