The ATPases of activated and relaxed rabbit psoas myofibrils were studied by the rapid flow quench method in a solvent of near physiological pH and ionic strength. Both types of myofibrils bind and hydrolyze ATP with transient kinetics very similar to those found with myosin. But the kcat of activated myofibrils was 100× that with the relaxed myofibrils. Relaxed myofibrils and myosin could not be distinguished kinetically.