The initial rates of ATP regeneration catalyzed by acetate kinase (ATP : acetate phosphotransferase; EC 2.7.2.1) from Bacillus stearothermophilus were measured under a wide range of MgADP(-), acetyl phosphate, MgATP(2-) and acetate concentrations. The experimental results shelved that the enzyme exhibited positive co-operativity for MgATP2(-) and no co-operativity for MgADP(-), acetyl phosphate and acetate. Furthermore, the initial rates of the forward reaction, in which MgATP(2-) and acetate were produced from MgADP(-) and acetyl phosphate, were inhibited by MgATP(2-) and acetate, and those of the reverse reaction were inhibited by MgADP(-) and acetyl phosphate. The initial rate data were correlated well by using the rate equation derived based on the assumption that the reaction obeys the general reaction scheme based on the Random Bi Bi mechanism, and on the previous experimental result that the enzyme behaves like a dimeric enzyme, even though it is a tetrameric enzyme.