The purpose of this study was, firstly, to investigate the intensity of exercise performance of highly trained ultra-endurance triathletes during the cycling portion of an Ironman triathlon, and, secondly, to examine the anaerobic threshold and its relationship to this performance. Following a peak oxygen consumption (VO(2peak)) test on a cycle ergometer to determine the heart rate (HR(Th,vent)) and power output (PO(Th,vent)) at the ventilatory threshold (Th(vent)), 11 highly trained male triathletes [mean (SEM) age 35.8 (1.6) years, body fat 11.7 (1.2)%. VO(2peak) 67.5 (1.0) ml x kg(-1) x min(-1)] who were participating in an Ironman triathlon, in random order: (1) cycled at their PO(Th,vent) (Bi(Th,vent)) until they were exhausted, and (2) cycled for 5 h at a self-selected intensity (Bi(SSI)). Cycling power output (PO), oxygen uptake (VO(2)), heart rate (HR) and blood lactate concentration ([La(-)](b)) were recorded at regular intervals during these trials, while performance HR was recorded during the cycling phase of the Ironman triathlon. Significantly greater (P < 0.05) values were attained during Bi(Th,vent) than during Bi(SSI) for PO [274 (9) compared to 188 (9) W], VO(2) [3.61 (0.15) compared to 2.64 (0.09) l x min(-1)], and [La(-)](b) [6.7 (0.8) compared to 2.8 (0.4) mmol x l(-1)]. Moreover, mean HR during the Ironman triathlon cycle phase [146.3 (2.4) beats.min(-1); n=7] was significantly greater than mean HR during Bi(SSI) [130 (4) beats x min(-1)], and significantly less than mean HR during Bi(Th,vent) [159 (3) beats x min(-1); all P < 0.05]. However, HR during the cycle portion of the Ironman triathlon was highly related to (r = 0.873; P < 0.05) and not significantly different to HR(Th,vent) [150 (4) beats x min(-1)]. These data suggest that ultra-endurance triathletes cycle during the Ironman triathlon at a HR intensity that approximates to HR(Th,vent), but at a PO that is significantly below PO(Th,vent).