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1. Heat production, tension development and phosphorylcreatine (PC) splitting have been measured simultaneously during isometric contractions of iodoacetate-poisoned frogs' sartorii at 0° C. The muscles were stimulated to produce a series of 30 twitches or a 10 sec tetanus or a 30 sec tetanus.

2. Of the several possible methods of calculating PC breakdown the best appears to be the one based on the assumption that PC/(total creatine) was originally the same in the two muscles of a pair.

3. The difficulty of expressing PC break-down, heat production etc. in terms of the muscle's size is demonstrated.

4. Several artifacts are discussed, including the heat produced by the stimulus and the possible role of reactions other than PC splitting.

5. Even when these have been substantially eliminated there remains the rather intractable statistical problem of establishing a functional relationship between heat produced and PC split when both of these variables contain errors, so that ordinary regression analysis is misleading. It is shown that this problem can be dealt with satisfactorily by introducing other instrumental variables that can be determined experimentally.

6. The final conclusion is that the in vivo enthalpy of hydrolysis of PC is about 10·6 kcal/mole, and is the same in twitches and in tetani.