During ATP hydrolysis, myosin or subfragment 1 catalyzes the exchange of oxygens between water and phosphate, so that on average, each product Pi molecule contains more than one oxygen atom derived from water. Using quenched-flow techniques, the exchange process in both ATP and Pi was studied in the rabbit skeletal muscle subfragment 1 ATPase. The exchange in protein-bound ATP (M*.ATP) and protein-bound ADP.Pi (M**.ADP.Pi) was followed as a function of time. The pattern of exchange follows closely a model in which M*.ATP + H2O and M**.ADP.Pi are interconverted directly, without any characterizable intermediate between these species. All oxygen atoms of Pi are equivalent with respect to loss to solvent, and elimination of water to re-form ATP occurs with a rate constant of 15 s-1 (at pH 8.0, I 0.015 M, 20 degrees C). This is the first direct measurement of the rate constant for the transformation of M**.ADP.Pi to M*.ATP. Oxygen exchange during steady state ATP hydrolysis in the presence of acto-subfragment 1 also fits well to this model, with actin reducing the time available for M*.ATP and M**.ADP.Pi to undergo exchange.