We have previously demonstrated that transdermal nicotine administration improves cycling time to exhaustion (Mündel & Jones, 2006) but does not influence 1h cycling time-trial performance (Mündel et al. 2010) in a moderate (~20°C) ambient temperature. However, evidence from similarly acting drugs suggests that any performance improvements only occur at warmer ambient temperatures, when they also cause body temperature and cardiovascular function to be pushed closer to the limit of safety. Many competitive sporting events take place during the summer period or in warm climates. Nicotine is known to reduce skin blood flow and when combined with exercise this raises a serious concern over the safety of its use during exercise in a warm environment, where increasing skin blood flow and sweating are the primary routes of heat loss, possibly placing an athlete at greater risk of developing a heat illness. Therefore, ten trained, male cyclists cycled for 60 min at 55% of their maximum power output followed by a work-dependent time-trial at 75% of their maximum power output (~30 min) on four occasions. Twice trials were conducted in 18°C and twice in 30°C and in each environment they were administered placebo and nicotine via transdermal patch. Values are mean ± standard error, compared by repeated measures ANOVA where p<0.05 signifies significance. Ambient temperature affected time-trial performance, such that both time to complete the set work was faster (4±1min) and mean power output was higher (24±8W) in 18°C than 30°C. Forearm blood flow (venous occlusion plethysmography) values increased from 18°C to 30°C (6±1ml/100ml/min-1) and decreased from placebo to nicotine (3±1ml/100ml/min-1). Mean weighted (12-site) skin temperature values were lower in 18°C than 30°C (3.7±0.3°C) and higher with nicotine than placebo (0.5±0.2°C). Core (gastro-intestinal) temperature values were lower in 18°C than 30°C (0.4 ± 0.1°C) and higher with nicotine than placebo (0.2±0.1°C). Total sweat loss and % body weight loss were lower in 18°C than 30°C (0.6±0.1L and 0.7±0.2%). Heart rate was lower in 18°C than 30°C (8±1 beats/min-1). In summary, although the ambient temperature affected performance, nicotine did not. Further, nicotine decreased forearm blood flow and raised both skin and core body temperatures during exercise. Therefore, from these results it is concluded that under these conditions, nicotine is neither ergogenic nor ergolytic but does increase the risk for heat illness when administered during exercise coupled with heat stress.