Heat production and chemical change in tortoise muscle.

1. Measurements have been made of heat production and changes in levels of phosphorylcreatine (PC), ATP and lactic acid resulting from contraction of tortoise muscle under anaerobic conditions.2. The only significant chemical change found was a break-down of PC.3. The amount of heat produced per mole of PC split (-DeltaH) was 13.18 +/- 1.04 kcal/mole (mean and S.E. from thirty-five observations).4. (-DeltaH) is probably rather greater in tortoise than in frog muscle. The value of (-DeltaH) cannot easily be accounted for by the known processes accompanying PC splitting in either type of muscle.