This study evaluated the effect of time of day on performance of high-intensity, constant-power cycle ergometry by both men and women. Subjects performed all-out cycle ergometer tests in the morning and in the afternoon in randomized order. For all tests, work rate was a constant 5.0 W.kg-1 (women, n = 6) or 6.0 W.kg-1 (men, n = 8). Total work performed was 9.6% greater in the afternoon (mean +/- SE, 348.8 +/- 40.6 J.kg-1) compared to the morning (318.2 +/- 39.5 J.kg-1). The greater amount of work in the afternoon was associated with a 5.1% higher aerobic power and a 5.6% larger anaerobic contribution. There was no interaction between gender and the effect of time of day on the aerobic or anaerobic contributions. These results provide evidence of a circadian rhythm in aerobic and anaerobic responses to high-intensity short-duration exercise, in women as well as in men.