Fluorescence stopped-flow experiments were performed to elucidate the elementary steps of the ATPase mechanism of scallop heavy meromyosin in the presence and in the absence of Ca2+. ATP binding and hydrolysis, as monitored by the change in tryptophan fluorescence, appear to be Ca2+-insensitive, whereas both Pi release and ADP release are markedly suppressed in the absence of Ca2+. Rate constants for Pi release are 0.2 s-1 and 0.002 s-1 and for ADP release are 6 s-1 and 0.01 s-1 in the presence and in the absence of Ca2+ respectively. Ca2+ binding to the specific site of the regulatory domain is rapid and its release occurs at 25 s-1, consistent with the time scale of a twitch of the striated adductor muscle. Nucleotide binding is a multi-step process requiring a minimum of three states. In such a model Ca2+ controls the rate of conformational changes at the active site in both the forward and the reverse direction, leading to a large dependence of the rate of nucleotide release, but a lesser effect on the overall equilibrium position. The kinetic trapping of nucleotides and Pi at the active site, in the absence of Ca2+, appears to be a fundamental step in suppressing the interaction of the myosin head with the thin filaments in relaxed molluscan muscle.