The effect of prior exercise (PE) on subsequent maximal short-term power output (STPO) was examined during cycling exercise on an isokinetic ergometer. In the first series of experiments the duration of PE at a power output equivalent to 98% maximum O2 uptake (VO2max) was varied between 0.5 and 6 min before measurement of maximal STPO. As PE duration increased subsequent STPO fell to approximately 70% of control values after 3-6 min. In series ii the effect of varying the intensity of PE of fixed 6-min duration was studied in five subjects. After PE less than 60% VO2max there was an increase of 12% in STPO, but after greater than 60% VO2max there was a progressive fall in STPO as PE intensity increased, indicating a reduction of approximately 35% at 100% VO2max compared with control values. In series iii we examined the effect on STPO of allowing a recovery period after a fixed intensity (mean = 87% VO2max) of 6 min PE before measurement of STPO. This indicated a rapid recovery of dynamic function with a half time of approximately 32 s, which is similar to the kinetics of PC resynthesis and taken with the other findings suggests the dominant role that PC exerts on the STPO under these conditions.