1. The temperature-dependence of the maximum velocity of shortening and of the maximum rate of isometric tension development were examined in a rat fast twitch muscle, extensor digitorum longus (e.d.l.), and a slow twitch muscle, soleus, in vitro and with direct stimulation. The temperature range was 35-20 degrees C.2. The maximum velocity of shortening and the maximum rate of tension development decreased with cooling in both muscles. The decrease was such that their logarithms were linearly related to the reciprocal of the absolute temperature over the temperature range of 35-20 degrees C in e.d.l. and 35-25 degrees C in soleus.3. The calculated Arrhenius activation energy for maximum velocity of shortening was around 40-45 kJ in both muscles. The activation energies for maximum rate of tension development were 48 kJ in e.d.l. and 56 kJ in soleus. Similar analyses made on the time to peak and time to half-relaxation of the isometric twitch showed that their activation energies were higher, between 60-80 kJ, in both muscles.4. The results are examined in relation to biochemical findings and discussed in relation to A. F. Huxley's (1957) cross-bridge theory.