Work per cycle was calculated in isolated frog sartorius muscle by measuring force as the activated muscle was subjected to sinusoidal length changes. Work per cycle was calculated from the area of the loop formed when force was plotted against length. Measurements were made at 20 degrees C using physiological solutions with high pHe (7.9 or low pHe (6.6). Net work done per cycle was positive when the muscle was activated during the shortening phase of the length cycle. Maximum work done increased as excursion amplitude increased and was about 17Jkg-1 at a strain of 12% (i.e. Lo +/- 6%) at a cycle frequency of 2 Hz. Maximum power was 35 W kg-1 and was about 25% less at the lower pHe. Power measured in this way is much less than the value calculated from force-velocity curves. However, power calculated from force-velocity curves neglects the time that the muscle must be inactive during locomotion. Thus the measurements of the present study are realistic relative to actual mechanical power output during locomotion.