This study examined the effects of active warm up on thermoregulatory responses and intermittent-sprint cycle performance in hot conditions (35.5±0.6°C, RH 48.7±3.4%). Eight trained males performed a 36-min, intermittent-sprint test (IST) after no (WUP 0), 10-min (WUP 10) or 20-min warm up (WUP 20). The IST contained 2-min blocks consisting of a 4-s sprint, 100s active recovery and 20s passive rest. Twice during the IST, there was a repeated-sprint bout (RSB) comprising five, 2-s sprints separated by ∼20s. There were no significant differences between trials for mean work (3870±757 versus 4028±562 versus 3804±494Jsprint −1 ), peak power (W) or work decrement (%). However, mean work was significantly less in RSB2 than RSB1 for WUP 20 only (P<0.05). Plasma lactate was significantly higher after active warm up (WUP 20=WUP 10>WUP 0; P<0.05), but not significantly different between conditions following either RSB. Rectal temperature (T re ) was significantly higher after active warm up (37.0±0.3 versus 37.3±0.3 versus 37.7±0.1°C for WUP0, WUP10 and WUP20, respectively) and throughout the IST. The longer active warm up resulted in a greater increase in T re and was associated with a decrease in short-term repeated-sprint ability (with incomplete recovery), but not prolonged, intermittent-sprint performance in the heat. As active warm up did not improve performance (<40min), team-sport athletes may minimise changes in T re (and the likelihood of heat illness) by avoiding excessive warm up when competing in the heat.