This paper presents the results of three experiments to determine the effect of sleep deprivation and physical fatigue on ratings of perceived exertion (RPE) for exercise of short (30 s) and long (15-50 min) duration. In the first, 12 male subjects walked on a treadmill, 50 min every 3 h, at 28% of their VO2max for 60 h without sleep. The RPE, recorded at the mid-point of each exercise bout, increased progressively with sleep loss. In the second experiment, when 12 female subjects performed ten 30-s bicycle exercise bouts before and after 54 h without sleep, there was no significant increase in RPE. In the third experiment, which examined the effect of physical fatigue on RPE, seven males performed two sets of ten 30-s bouts of cycle exercise before and after treadmill exercise at 70% of their VO2max to voluntary exhaustion. In one set, RPE was recorded; in the other, power output was measured. During the treadmill running, increases in RPE were observed and attributed to fatigue. This same fatigue caused only small changes in RPE and power output for the cycle exercise. The combined data suggest that where the exercise is of several minutes' duration, RPE is increased by sleep deprivation and by physical fatigue. Where the exercise period is as short as 30 s, sleep deprivation has no effect and physical fatigue causes only a small change in the perception of exercise intensity.