&NA;The actin dependence of the rate and magnitude of the initial phosphate burst was measured using both quench‐flow and stopped‐flow kinetic techniques. These studies revealed that even at high actin concentrations the magnitude of the phosphate burst was a significant fraction of the magnitude that exists in the absence of actin. Furthermore, it was shown that the rate of the burst rises rapidly as a function of the actin concentration. Detailed modeling with the four‐state model revealed that if the predicted Vmaxis constrained to be approximately equal to the extrapolated value (double reciprocal plot) and if the apparent dissociation constant of subfragment‐1 to actin divided by the apparent activation constant of the actin‐activated myosin ATPase activity (Kbinding/KATPase) is constrained to be considerably different from one, then the model is unable to simultaneously account for the ATPase activity and the rate and magnitude of the initial inorganic phosphate burst. (Circulation Research1989;65:515‐525)