Isolated rat and mouse extensor digitorum longus (EDL) and soleus muscles were studied under isometric and isotonic conditions at temperatures from approximately 8 degrees -38 degrees C. The rate constant for the exponential rise of tension during an isometric tetanus had a Q10 of approximately 2.5 for all muscles (corresponding to an enthalpy of activation, delta H = 66 kJ/mol, if the rate was determined by a single chemical reaction). The half-contraction time, contraction time, and maximum rate of rise for tension in an isometric twitch and the maximum shortening velocity in an isotonic contraction all had a similar temperature dependence (i.e., delta H approximately 66 kJ/mol). The Mg++ ATPase rates of myofibrils prepared from rat EDL and soleus muscles had a steeper temperature dependence (delta H = 130 kJ/mol), but absolute rates at 20 degrees C were lower than the rate of rise of tension. This suggests that the Mg++ ATPase cycle rate is not limiting for force generation. A substantial fraction of cross-bridges may exist in a resting state that converts to the force-producing state at a rate faster than required to complete the cycle and repopulate the resting state. The temperature dependence for the rate constant of the exponential decay of tension during an isometric twitch or short tetanus (and the half-fall time of a twitch) had a break point at approximately 20 degrees C, with apparent enthalpy values of delta H = 117 kJ/mol below 20 degrees C and delta H = 70 kJ/mol above 20 degrees C. The break point and the values of delta H at high and low temperatures agree closely with published values for the delta H of the sarcoplasmic reticulum (SR) Ca++ ATPase. Thus, the temperature dependence for the relaxation rate of a twitch or a short tetanus is consistent with that for the reabsorption rate of Ca++ into the SR.