The effect of time of day on ratings of perceived exertion (RPE) at various intensities of cycling exercise, both below and above the ventilatory threshold, was studied in 32 subjects, 18 to 35 years of age. The ventilatory threshold occurred at the same (p greater than .05) mean (+/- SD) work rate in the morning (110.6 +/- 27.0 watts) and in the afternoon (111.9 +/- 23.9 watts) and was perceived as equally strenuous (p greater than .05) in the morning (RPE = 13.8 +/- 2.4) and in the afternoon (RPE = 13.6 +/- 2.8). At intensities below the ventilatory threshold, RPE was the same (p greater than .05) in the morning and in the afternoon; above the ventilatory threshold, RPE was lower (p less than .05) in the morning. We conclude that, during incremental submaximal cycling exercise above the ventilatory threshold, a particular work rate is perceived as less strenuous in the morning than in the afternoon. About 20% of this difference in RPE is explained by lower ventilatory demands in the morning.