Isolated respiratory muscle endurance training (RMT) can prolong constant-intensity cycling performance. We tested whether RMT affects O2 supply during exercise, i.e. whether the partial pressure of oxygen in arterial blood (Pa,O2) and/or its oxygen saturation (SaO2) are higher during exercise after RMT than before. A group of 28 sedentary subjects were randomly assigned to either an RMT (n = 13) or a control group (n = 15). The RMT consisted of 40x30 min sessions of normocapnic hyperpnoea. The control group did not perform any training. Breathing and cycling endurance time as well as PaO2 and SaO2 during cycling at a constant intensity of 70% maximum power output were measured before and after the RMT or the control period. Mean breathing endurance increased significantly after RMT compared to control [RMT 5.2 (SD 2.9) vs 38.1 (SD 6.8) min, control 6.5 (SD 5.7) vs 6.4 (SD 7.6) min; P < 0.01], as did mean cycling endurance [RMT 35.6 (SD 11.9) vs 44.0 (SD 17.2) min, control 32.8 (SD 11.6) vs 31.4 (SD 14.4) min; P<0.05]. The RMT did not affect PaO2 which ranged from 11.6 to 12.3 kPa (87-92 mmHg), and SaO2 which ranged from 96% to 98% throughout all tests. In conclusion, RMT substantially increased breathing and cycling endurance in sedentary subjects. These changes, however, cannot be attributed to increased O2 supply, as neither PaO2 nor SaO2 were increased during exercise after RMT.