The rate and extent of inactivation of myofibrillar EDTA-ATPase in rabbit psoas fiber bundles at various temperatures between 3 and 42' and a t pH values between 5.5 and 7.0 have been studied by measuring the EDTA-ATPase activity. The purification of psoas myofibrils with Triton X-100 permitted the direct measurement of changes in the myofibrillar ATPase activity of psoas fiber bundles stored under various conditions. The myofibrillar ATPase, when the fiber bundles are stored in the medium a t 3' and pH 5.5, are quite stable. During storage a t 3O for 72 hr, it has been found that myofibrillar ATPase activity remained almost constant between pH 5.5 and 7.0. However, at higher temperatures, the rate of thermal inactivation is increased in the more acidic systems. From the results it has been concluded that the effect, of pH on the loss of myofibrillar ATPase is negligible if the fiber bundles are stored near 0' and that the most important factor influencing the denaturation is the temperature at which the fiber bundles are stored.A naturally occurring phenomenon of some interest is the so-called PSE (pale, soft, and exudative), or watery condition in some types of pig muscle. The condition is characterized by a very rapid pH fall postmortem, and by soft, pale, and watery musculature in the post-rigor state (Briskey and Wismer-Pedersen, 1961;Bendall and Lawrie, 1964;Briskey, 1964). Whale, beef, and rabbit muscles, subjected to high temperature rigor, also develop wateriness, so the condition is of general occurrence under these conditions and is not peculiar to pigs (Bendall and WismerPedersen, 1962;Bendall and Lawrie, 1964). Penny (1967) and Yasui e t al. (1973) were able to show that myosin B (natural actomyosin) and myosin prepared from rabbit muscle were considerably denatured under these conditions. These authors demonstrated that both types of protein were sensitive to temperature and pH in the range to be expected in a cooling carcass, i.e., 36-38' and a pH of 6.0 or below within 1 hr of death in those samples exhibiting the highest rate of glycolysis. We can therefore attribute the PSE phenomenon principally to protein denaturation at high temperature and low pH.In order to investigate what actually occurs in intact muscle during rigor, it is desirable to use a highly organized muscle model whose structural features are similar to that of intact muscle. Yasui et al. (1973) studied the denaturation of myosin in glycerol-treated fiber bundles, which were fixed t o application rods a t rest length, using extractability and Ca*+-ATPase activity as criteria. Their results indicated that the protein in the fiber bundles was more stable than the isolated protein molecules.T o confirm and extend the conclusion derived from their study, the preferred approach was to investigate directly the myofibrillar ATPase under the conditions of temperature and pH encountered in practice. This has not been done so far for the following two reasons. (1) When the washed myofibrils are exposed to severe conditions, e.g., high...