We tested the hypothesis that isolated adult rat heart cells could be depleted of most of their ATP without undergoing contracture. Two strategies for ATP depletions were employed. First, cells were exposed to a high level of rotenone plus FCCP. The cells lost 90% of their ATP within 3 min without change in sarcomere length before undergoing contracture. Even though ATP levels were so low, glycolysis from glycogen was maximally activated at this time. Second, cells exposed to repeated cycles of acidic anoxia were depleted of 77% of their ATP without change in sarcomere length and remained rod-shaped when restored to normoxia and neutral pH. The hypothesis was thus confirmed. The results support the previously developed concept that ATP decline in cells can be synchronous, with a similar decline in all cells, or asynchronous, with a sudden decline in different cells at different times. Whether the decline is synchronous or asynchronous depends on the conditions of metabolic impairment. This concept can explain the pattern of ATP decline observed in whole hearts during ischemia, and also the mechanism by which glycolytic ATP appears to protect against contracture.