Passively elevating muscle temperature increases the maximal power output developed during cycle exercise (Sargeant et al. 1987). This is probably due to a greater rate of cross-bridge cycling that will require a greater rate of ATP turnover (Edwards et al. 1972). Such findings have not, however, been demonstrated in humans. The aim of this study was to investigate the effect of elevated muscle temperature on the rate of skeletal muscle ATP turnover during the development of maximal power output. Following local ethics committee approval six male subjects (age 25 ± 2 years, height 182 ± 3 cm, mass 77.1 ± 4.5 kg; means ± S.E.M.) volunteered to perform a 6 s maximal sprint on a mechanically braked cycle ergometer under conditions of normal (N) and elevated muscle temperature (ET). Muscle temperature (T_m) was elevated through passive heating of the legs. Prior to exercise, T_m of the vastus lateralis was measured at a depth of ~2-3 cm using a flexible thermistor. Biopsies were obtained from the vastus lateralis muscle before and immediately after each sprint and analysed for content of ATP, PCr and lactate from which anaerobic ATP turnover was calculated (see Spriet, 1995). Temperature co-efficient (Q₁₀) values were calculated for power output variables and ATP turnover. Statistical analyses were performed using two-way repeated measures ANOVA and paired t tests where appropriate. Tm was 3.0 ± 0.3°C higher in ET compared with N (P<0.05). Maximal power output, corrected for flywheel inertia (Lakomy, 1986) and pedal rate both increased by 258 ± 110 W (Q₁₀ of 2.2) and 22 ± 6 revs min-1 (Q₁₀ of 1.6) respectively with elevated temperature (P<0.05). During exercise the rate of ATP turnover for both PCr hydrolysis and glycolysis were greater during ET (Q₁₀ of 3.8 and 1.7, respectively; P<0.05). Consequently, the rate of anaerobic ATP turnover was greater during ET compared with N, with a Q₁₀ of 2.9; Table 1; P<0.05). We have demonstrated that the higher power output obtained with passive heating of the muscles was achieved at least partly through an increased rate of anaerobic ATP turnover.