1. An isolated skeletal muscle preparation is described which has been used to study the efflux of enzymes in response to contractile activity, metabolic poisons and detergent treatment.

2. In both fast and slow muscles contractile activity caused a release of lactate dehydrogenase and creatine kinase that reached a peak 1–2 h after the end of stimulation. There was very little release during the 30 min stimulation period whether the muscles were under aerobic or hypoxic conditions.

3. Incubation of the muscles with cyanide and iodoacetate caused a similar delayed release of enzyme.

4. Disruption of cell membranes with detergent treatment caused an entirely different and very rapid pattern of enzyme release.

5. Enzyme release from the fatigued isolated muscle preparations appears to be initiated as a consequence of phosphorylcreatine or ATP depletion. The relevance of this to release of muscle enzymes after activity in vivo is discussed.

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