1. High-energy phosphate metabolism and energy liberated as heat and work were measured in 3 sec tetani of frog sartorius muscles at 0 degrees C.2. Three contraction periods were studied: (a) shortening at near-maximum velocity for 0.3 sec from sarcomere length 2.6 to 1.8 mum, beginning after 2 sec of isometric stimulation, (b) the 0.7 sec isometric period immediately following such rapid shortening, (c) the period from 2 to 3 sec in an isometric tetanus at sarcomere length 1.8 mum.3. There were no significant changes in levels of ATP, ADP or AMP in any contraction period. The observed changes in inorganic phosphate and creatine levels indicated that the only significant reaction occurring was phosphocreatine splitting.4. The mean rate of high-energy phosphate splitting during rapid shortening, 0.48 +/- 0.24 mumole/g.sec (mean +/- s.e. of mean, n = 29; ;g' refers to blotted muscle weight), was not significantly different from that in the 1 sec period in the isometric tetanus, 0.32 +/- 0.11 mumole/g.sec (n = 17). The mean rate in the post-shortening period, 0.71 +/- 0.10 mumole/g.sec (n = 22), was greater than that in the 1 sec period in the isometric tetanus, and this difference is significant (P < 0.02, t test).5. A large quantity of heat plus work was produced during the rapid shortening period, but less than half of this could be accounted for by simultaneous chemical reactions. The unexplained enthalpy production was 6.5 +/- 2.6 mJ/g (mean +/- s.e. of mean). No significant unexplained enthalpy was produced in the 1 sec period in the isometric tetanus.6. In the post-shortening period the observed enthalpy was less, by 6.2 +/- 2.6 mJ/g, than that expected from the simultaneous chemical reactions.7. The results are interpreted in terms of an exothermic shift in the population of cross-bridge states during rapid shortening. It is suggested that a relatively slow subsequent step prevents many of these cross-bridges from completing the cycle and splitting ATP until after the end of shortening.