Ventilatory and lactate thresholds have been proposed as tools to establish the highest steady-state intensity sustainable during prolonged physical exercise. The purposes of this study were to clarify whether the intensity at the ventilatory threshold could be sustained during prolonged high-intensity exercise and if the corresponding work rate, pulmonary ventilation, and blood lactate concentration could also be maintained. Fifteen young and healthy male subjects were submitted to a ˙VO2maxtest on ergocycle and a 90-min high-intensity ergo-cycle endurance exercise test. During the 90-min exercise test, subjects were able to maintain an intensity corresponding to a heart rate 5 beats·min-1lower than that predetermined from the ventilatory threshold. Heart rate, FeO2, and FeCO2were stable during the period from 20 to 80 min. ˙VO2was constant from 30 to 80 min, while work output, pulmonary ventilation, blood lactate, and˙VCO2decreased significantly over the 90-min performance. These results show that physiological parameters near the ventilatory threshold are not interchangeable and that some cannot be used to monitor high-intensity long term exercise. Moreover, they clearly demonstrate that the blood lactate concentration fluctuates substantially during a 90-min endurance performance and cannot predict the highest work intensity that can be sustained during prolonged exercise without fatigue. However, heart rate and ˙VO2at the ventilatory threshold seem to be more suitable markers for that purpose.