Myothermic determinations of the energetics of rat fast twitch extensor digitorum longus (EDL) and slow twitch soleus (SOL) reveal that at birth the total energy production is similar but by 41 days postpartum, the total energy production of the EDL is much greater, The higher energy liberation of EDL also exists in the mature mouse. The differences in total energy production result from higher values for both tension-independent and tension-dependent heat in the EDL. The heat-load relations of the two muscles are similar and were temperature- and initial length-dependent. Recovery in EDL is glycolytic in nature whereas mitochondrial oxidation predominates in the SOL. Quantitative differences exist between mature mammalian fast and slow twitch muscles that result from basic differences in excitation-contraction coupling, maximum velocity of shortening, and the muscle's enzymatic characteristics.