The effect of either an active warm up (AWU) or a passive warm up (PWU) on performance and energy system contribution in a moderately hot ambient environment (30 degrees C, 50% Relative humidity) was investigated. The AWU procedure involved exercise at 100% VO2 max followed by four 30 sec efforts at 100% of VO2 max and the PWU involved the application of heat pads to the quadriceps for 60 min. Following either warm up procedure, (randomized cross over design) a 60 s supramaximal test on a Monark bicycle ergometer was performed. The AWU procedure elicited the greatest average power output (0-30 s: 669 +/- 28 W vs 645 +/- 30 W, p < 0.05) and (0-60 s 529 +/- 20 W vs 513 +/- 20 W, p < 0.05), the greatest peak power output (917 +/- 45 W vs 880 +/- 46 W, p < 0.01) and VO2 peak during the initial 30 s (1.4 +/- 0.1 vs 1.2 +/- 0.1 l.min-1, p < 0.01) and over the duration of the 60 s test (3.2 +/- 0.1 vs 2.9 +/- 0.2 l.min-1, p < 0.05). The estimated anaerobic energy release was not significantly different between the trials. The performance benefits of an AWU occurred within the first 30 s of exercise as there was no significant differences in average power output or VO2 peak in the final 30 s of exercise. In conclusion, an AWU enhances sprinting performance in a moderately hot ambient environment, despite a higher rectal temperature than a PWU. The increased power output is derived from greater oxygen consumption in the initial stages of exercise.