1. The heat+work (h+w) and the changes in the levels of creatine, phosphocreatine and ATP were determined for 1, 2, 5, 10 and 15 sec isometric tetani at 0 degrees C under aerobic conditions. The change in the sum of inorganic phosphate and glucose-1-phosphate and glucose-6-phosphate was measured also. The changes in the levels of all these chemicals and of lactate were measured in muscles stimulated for 15 sec under anaerobic conditions. 2. The lactate measurements and a comparison of the results for aerobic and anaerobic conditions showed that during a 15 sec tetanus there was a negligible amount of resynthesis of ATP from reactions other than the creatine kinase reaction. 3. For all durations of stimulation, except 1 sec, a significant part of the h+w could not be explained by the energy from ATP splitting and the creatine kinase reaction. The existence of an unexplained part of the h+w confirms earlier findings. 4. On the basis of its time course, the h+w was divided into the stable part and the labile part. The energy from the observed chemical reactions was always sufficient to account for the stable part of the h+w. 5. Early in the tetanus the unexplained energy is less than the labile part of the h+w. At the end of a 15 sec tetanus the total amounts of unexplained energy and labile h+w are equal. For this reason and others which are discussed it is probable that there is a close relationship between them.