The rate constants of a series of elementary steps in the H-meromyosin (HMM) Mn²⁺-ATPase [EC 3. 6. 1. 3] and acto-HMM Mn²⁺-ATPase reactions were determined in 0.1M KCl at 5°C. We found that the rate-limiting step in the HMM Mn²⁺-ATPase reaction was the liberation of ADP from HMM•ADP, of which the rate constant was estimated to be 0.17 s^-1. All the results obtained with the acto-HMM Mn²⁺-ATPase reaction could be quantitatively explained by a modified Lymn-Taylor mechanism (see Fig. 15). The second-order rate constant for the dissociation of acto-HMM induced by ATP was 3.0×10⁵M^-1•s^-1, and that for the P₁, burst in the acto-HMM ATPase reaction was 1.7×10⁵M^-1•s^-1. The second-order rate constant for the binding of HMMADP with F-actin was 0.25 s^-1•mg^-1•ml, and the rate-limiting step in the acto-HMM Mn²⁺-ATPase reaction was the conversion of HMM^ADP_P into HMM•ADP plus P₁, when the F-actin concentration was high.