ABSTRACT Background: For many decades researchers have studied physiological characteristics of endurance athletes, such as runners, cyclists, cross country skiers and triathletes. Maximal oxygen uptake (VO2max) is often described as the upper limit of endurance capacity. However, in hilly long distance triathlon, such as AXtri and NXtri, other test results may be much better predicators of race performance in the cycling event. Purpose: To examine the physiological determinants in two of the world s most extreme triathlon competitions. Methods: 16 (sixteen) athletes (13 boys, 3 girls) who had all been given a slot in the two triathlon races Aurlandfjellet Extreme Triathlon and Norseman Extreme Triathlon, were tested focusing on power output at lactate threshold, maximal oxygen consumption/uptake and peak power output. We compared the physiological test results with the athletes performance during the cycling in the two long distance triathlons. Results: We found that the athletes, who performed at the highest level during the cycling event in the competitions, also had the best values considering power output at lactate threshold (LT) and peak power output (PPO). The single variable that correlated best with cycling performance was power output at LT (r = 0.83, p = 0,0001) and peak power output (r = 0,78, p = 0,0007). Maximal oxygen uptake (VO2max) did not correlate significantly with cycling performance (r = 0,47, p = 0,07). Conclusion: The results of this study demonstrate that the power output at LT and peak power ouput is important for performance during the cycling event in long distance triathlon. Key words: Triathlon, long distance triathlon, Norseman Extreme Triathlon, Aurlandsfjellet Extreme Triathlon, power output, peak power output, lactate threshold, anaerobic threshold, maximum oxygen uptake, training.