Effects of divalent metal cations on the heavy meromyosin ATPase [EC 3.6.1.3] reaction were studied by measuring the ATP-induced ultraviolet absorption difference spectrum at the pre-steady and steady states, and the Pi-liberation at the steady state. Effects of MgCl2 concentration on apparent values of the parameters, the maximum velocity and Michaelis constant of the ATPase, the velocity of formation of the difference spectrum, and the molar difference absorption coefficient were investigated. From the analysis of the results, the role of divalent cation on the ATPase reaction has been elucidated according to a reaction mechanism which involves two Michaelis complexes, ES and EMS, where M represents the divalent metal cation. Rate constant of the step from EMS to give product, k2′, and the dissociation constant of EMS to EM and S, K2′, were determined. In the presence of any one of Mg2+, Co2+, Mn2+ or Ca2+, logk2′ vs. logK2′ plot was linear. The plot of logk2′ vs. reciprocal of ionic radius of the cation was also linear. Results indicate that the smaller the ionic radius of cation, the more strongly ATP is bound by EM and the more slowly product is formed from EMS. The pH dependency of an apparent dissociation constant of EMS to ES and M yielded a second order dissociation curve of pKa=7.3. Two imidazole groups are suggested to participate in the binding of the divalent cation.