Previous work has demonstrated that exercise performance varies with time of day. The aim of this study was to investigate the influence of time of day on measures of anaerobic power and anaerobic capacity. Twelve male subjects, aged 18-22 years, performed a stair run test, a standing broad jump and the Wingate Anaerobic Test on twelve separate occasions. These were at 02:00, 06:00, 10:00, 14:00, 18:00 and 22:00 hours, duplicate measurements being obtained at each time point. Subjects' diet and activity prior to exercise and the laboratory temperature were controlled. Pre-exercise rectal temperature was measured on each occasion. The rectal temperature data conformed to a cosine function: its peak occurred at 18:11 hours and the peak to trough variation was 0.76 degrees C (p < 0.001). There was a rhythm in performance on the stair run and the broad jump tests, in phase with the curve in rectal temperature. Results for peak power and mean power production on the Wingate test did not display a significant circadian rhythm. The stair run and broad jump tests seem to be more sensitive to circadian rhythmicity than does the Wingate Anaerobic Test.