Energetics of relaxation in frog muscle.

1. Changes in the levels of phosphocreatine (PCr) and ATP during relaxation after tetanic contraction have been observed and compared with the heat + work production during the same period.2. The results were the same using 2 sec or 15 sec tetani and using 15 or 30 Hz stimulation.3. During relaxation the mean chemical change (of ATP + PCr) was -0.105 +/- 0.078 mumol/g muscle (n = 53). This change is less than would be required to explain the heat + work produced.4. The PCr split during both 2 sec and 15 sec tetani (up to the last stimulus) was less than would be required to explain the heat + work produced. This confirms previous experiments.5. It is concluded that the discrepancy between chemical change and heat + work production does not diminish during relaxation but actually increases.