Mechanical work, ATP, ADP, PC, free creatine and lactate concentrations were determined on IAA poisoned frog sartorii tetanically stimulated in humidified N2 at 10 degrees C in isotonic conditions (0.25 or 0.45 Po). Tetanus duration was 0.35 s, number of tetani was varied from 0 (rest) to 25 (exhaustion). The mechanical work performed per mole ATP + PC split (W*P) amounted on the average to 16.7 kJ/mol. It was observed, however, that W*P increased from about 13 to about 24 kJ/mol with decreasing ATP concentration from about 2 (resting value) to about 1 mumol/g and that this decrease in ATP was associated with a decrease of the shortening (and relaxation) speed of the muscle to about 30% of the values observed on the first tetanus. It is concluded that the thermodynamic efficiency of muscle contraction, calculated from the ratio of WP* (measured) to the thermodynamic affinity (free energy change) of ATP hydrolysis (estimated) increases from about 0.3 to about 0.5 with decreasing ATP concentration and shortening speed.