BACKGROUND: Smoking has a thermogenic effect and is associated with low physical performance. Nevertheless, a direct, quantitative effect of acute smoking on exercise heat tolerance has not been reported. METHODS: Sixteen healthy young male volunteers, eight cigarette smokers, and eight non-smokers participated in the study. All subjects performed a maximal oxygen consumption test (VO2max) and a standardized heat tolerance test (HTT) after at least 12 h without smoking under the following conditions: no nicotine exposure, 10 min after nicotine exposure (2 mg nicotine lozenge), and 10 min after smoking two cigarettes (0.8 mg nicotine in each cigarette, smokers only). RESULTS: There was no significant effect of nicotine exposure on physiological performance and heat tolerance in the non-smokers group. In the smokers group, cigarette smoking, but not nicotine ingestion, resulted with higher heart rate (by 9+/-9 bpm) at the end of the HTT (p<0.05). Moreover, both smoking and nicotine ingestion increased smokers\' rectal temperature at the end of the HTT (by 0.24+/-0.16 degrees C and 0.21+/-0.26 degrees C, respectively, p<0.05) and were associated with higher sweat rate during the HTT (by 0.08+/-0.07 g/h and 0.06+/-0.08 g/h, respectively, p<0.05). Heart rate variability (HRV) analysis also revealed a higher LF/HF (low frequency/high frequency) ratio after exposure to nicotine and smoking in the smokers group compared with no exposure (2.13+/-2.57 and 2.48+/-2.76, respectively, p<0.05), indicating a higher sympathetic tone. CONCLUSIONS: According to this preliminary study, cigarette smoking and nicotine ingestion increase the physiological strain during a HTT in smokers. Acute smoking may, therefore, increase heat intolerance and the risk to heat injuries.