1. The normalized force or power developed during each of a series of repeated contractions was averaged over the entire cycle of activity and rest to provide a measure of performance referred to as sustained force or sustained power. We tested the hypotheses that compared with slow soleus muscles, fast extensor digitorum longus (EDL) muscles would attain lower maximum values of sustained force, but higher maximum values of sustained power. 2. During repeated contractions at a stimulation frequency of 150 Hz, forces and powers of soleus and EDL muscles of mice were determined in situ at 35 degrees C. The train rate of repeated contractions was incremented every 5 min to increase duty cycle until a maximum value for sustained force or power was reached. 3. In one set of repeated contractions, each contraction was preceded by a quick stretch and thereafter muscle length was held constant. The stretch minimized active shortening of muscle fibres. Sustained force was calculated from force at constant length. The maximum sustained force developed by soleus muscles of 4.58 +/- 0.31 N cm-2 (mean +/- S.E.M.) occurred at a duty cycle of 0.48. Compared with soleus muscles, EDL muscles attained a lower (P less than 0.05) maximum value of 1.38 +/- 0.15 N cm-2 at a 0.35 duty cycle. 4. During isovelocity shortening contractions, the maximum value for sustained power developed by soleus muscles of 7.4 +/- 0.5 W kg-1 occurred at a duty cycle of 0.18. Compared with soleus muscles, EDL muscles achieved a significantly greater maximum value of 9.1 +/- 0.4 W kg-1 at a 0.21 duty cycle.