We investigated whether, during maximal exercise, intercostal muscle blood flow is as high as during resting hyperpnoea at the same work of breathing. We hypothesized that during exercise, intercostal muscle blood flow would be limited by competition from the locomotor muscles. Intercostal (probe over the 7th intercostal space) and vastus lateralis muscle perfusion were measured simultaneously in ten trained cyclists by near-infrared spectroscopy using indocyanine green dye. Measurements were made at several exercise intensities up to maximal (WRmax) and subsequently during resting isocapnic hyperpnoea at minute ventilation levels up to those at WRmax. During resting hyperpnoea, intercostal muscle blood flow increased linearly with the work of breathing (R2 = 0.94) to 73.0 +/- 8.8 ml min-1 (100 g)-1 at the ventilation seen at WRmax (work of breathing approximately 550-600 J min-1), but during exercise it peaked at 80% WRmax (53.4 +/- 10.3 ml min-1 (100 g)-1), significantly falling to 24.7 +/- 5.3 ml min-1 (100 g)-1 at WRmax. At maximal ventilation intercostal muscle vascular conductance was significantly lower during exercise (0.22 +/- 0.05 ml min-1 (100 g)-1 mmHg-1) compared to isocapnic hyperpnoea (0.77 +/- 0.13 ml min-1 (100 g)-1 mmHg-1). During exercise, both cardiac output and vastus lateralis muscle blood flow also plateaued at about 80% WRmax (the latter at 95.4 +/- 11.8 ml min-1 (100 g)-1). In conclusion, during exercise above 80% WRmax in trained subjects, intercostal muscle blood flow and vascular conductance are less than during resting hyperpnoea at the same minute ventilation. This suggests that the circulatory system is unable to meet the demands of both locomotor and intercostal muscles during heavy exercise, requiring greater O2 extraction and likely contributing to respiratory muscle fatigue.