Force-velocity data at different temperatures (range, 10-35¡C) from intact fibre bundles are analysed to determine the temperature dependence of the maximal mechanical power output of fast and slow rat muscles. At 35¡C, the maximal mechanical power was [similar]250 kW m-3 (= 250 µW mm-3) in fast (probably an underestimate) and [similar]100 kW m-3 in slow muscle. Within the more physiological temperature range (25-35¡C), the temperature coefficient (Q10) of maximum power was 2-2·5. In both muscles, the maximal power at 10¡C was only about 3-5 % of that at 35¡C, the decrease being particularly pronounced at temperatures below 20¡C (Q10 of 5-7).