1. The rate of rapid change in the UV spectra after adding ATP to H-meromyosin was measured over a range of high concentrations of ATP in 0.2 m KCl at pH 7.8 and 3.7°C. The values of Kf and Vf were 0.2 mM and 30 sec−1, respectively. 2. The rate of decomposition of the reactive myosin-phosphate-ADP complex, E, was measured using a rapid flow-dialysis method, and the following results were ADP obtained: (i) The rate constants of liberation of radioactive P1 and ADP from EPDP were similar to that of ATPase [EC 3.6.1.3] in the steady state at high ATP concentrations, but were much higher than that of ATPase at low ATP concentrations. (ii) They were unaffected by adding a large amount of non-radioactive ATP. (iii) The rate of liberation of ADP from the simple myosin-ADP complex, which was formed by mixing free enzyme and ADP, was too fast to be the rate-determining step in the ATPase reaction. 3. A typical initial burst occurred both at 0 and 20°C, after adding a large amount of ATP to myosin under conditions, in which all the active sites were occupied by ADP.