The relationship between exhaustion time ( t lim ) and the work performed at the end of constant-power exercises can be described by a linear relationship ( W lim = a + b t lim ) for work involving the whole body (eg cycling) or part of the body (eg knee extensions). The slope b in the equation is termed the critical power and has been proposed as an index of the capacity to perform work over a long period of time. The first objective of the present study was to compare the values of slopes b calculated from whole-body work of short duration, ie maximal and supra-maximal cycling exercises (slope b 1), with the values calculated from the same work, the durations of which were between 3.5 and 35 min (slope b 3), as in the protocols used by Scherrer and Monod (1960) for body-part work. Slope b 1 was significantly higher than slope b 3 in 10 subjects who performed 5 cycling exhausting exercises (60, 73, 86, 100 and 120% of maximal aerobic power (MAP) in watts). Exhaustion times corresponding to power outputs equivalent to b 1 and b 3 were equal to 29.0 ± 19.1 min and 48.6 ± 9.8 min respectively. Moreover, the exhaustion times at 60 and 73%,MAP were significantly correlated with slope b 3 (expressed in %MAP) but not with slope b 1. Consequently, slope b 3 should be considered as the critical power instead of slope b 1 as in some studies in the literature (Moritani et al, 1981). The second objective was to study the physiological significance of the critical power (slope b 3) of whole-body work (cycling). The workload that corresponded to a lactate steady state was not significantly different from b 3 (68.8 ± 6.0 vs 68.7 ± 6.3% MAP). Nevertheless, slope b 3 represents a workload corresponding to a slight but significant drift of heart rate or oxygen uptake. These results probably explain why b 3 is a power which can be maintained for a long time but not beyond about l h in an average subject.