Introduction: Exercise thresholds (e.g. ventilatory and lactate thresholds) are widely used in the field of sports science and demarcate the intensity domains of moderate, heavy and severe exercise. However, these thresholds should not be used interchangeably[1], also environmental factors (e.g. temperature) will influence the work rate at which these exercise thresholds will occur[2]. Since sports activities are often performed under hot environmental conditions, determination and interpretation of the exercise thresholds in these conditions is needed. Methods: Twelve physically active young men performed four exercise tests in total. Two ramp incremental exercise tests (30W.min-1), one in temperate conditions (18°C) (TEMP) and one in hot conditions (36°C) (HOT), were done to determine the ventilatory thresholds, i.e. gas exchange threshold (GET) and respiratory compensation point (RCP). GET was defined as the point where VCO2 increased disproportionate to VO2. RCP corresponded to the point where VE increased disproportionate to VCO2. Two step incremental exercise tests (80W+40W.3min-1) were executed to define the first and second lactate threshold, both in TEMP and HOT. The first lactate threshold (LT1) was determined as the point with the first increase in lactate concentration. The modified Dmax method was used to define the second lactate threshold (LTMOD). Exercise tests were performed in randomized order. Paired Samples T-Test was used for the statistical analysis in SPSS. Data are expressed as mean ± SD for n = 12. Results: Work rate at LTMOD in HOT is significantly different from LTMOD in TEMP (233 ± 33 vs. 246 ± 38 W; p = 0.019). Work rate at thresholds LT1, GET and RCP did not differ between HOT and TEMP. Nevertheless the heartbeat was significantly higher in HOT than in TEMP for threshold LT1 (135 ± 9 vs. 131 ± 10 bpm; p = 0.047), GET (154 ± 8 vs. 143 ± 10 bpm; p = 0.003) and RCP (168 ± 9 vs. 162 ± 10 bpm; p = 0.047). Maximal work rate in the step protocol was significantly lower in HOT than in TEMP (297 vs. 314W; p < 0.001), but not in the ramp protocol (363 ± 50 vs. 371 ± 45 W; p = 0.164). Work rate at GET in TEMP and HOT is significantly higher than at LT1, respectively 188 ± 37 vs. 167 21 W (p = 0.001) and 188 ± 34 vs. 161 ± 24 W (p < 0.001). Work rate at RCP is only higher than at LTMOD in HOT (243 ± 39 vs. 233 ± 33 W; p = 0.027). Conclusion: Although there is only at one exercise threshold (LTMOD) a significant lower work rate in HOT than in TEMP, the altered heart rate at the exercise thresholds must also be taken into account when analyzing and interpreting exercise tests. Therefore exercise tests must be performed in the environmental conditions as in which the peak performance has to be delivered. Additionally, ventilatory and lactate thresholds do not occur at the same work rate and thus should not be used interchangeably.