To elevate effects of carbon dioxide (CO2) retention by way of an increased respiratory load during submaximal exercise (150 W), the concentration changes of oxy- (DeltaHbO2) and deoxy-haemoglobin (DeltaHb) of active muscles and the brain were determined by near-infrared spectroscopy (NIRS) in eight healthy males. During exercise, pulmonary ventilation increased to 33 (28-40) L min-1 (median with range) with no effect of a moderate breathing resistance (reduction of the pneumotach diameter from 30 to 14 and 10 mm). The end-tidal CO2 pressure (PETCO2) increased from 45 (42-48) to 48 (46-58) mmHg with a reduction of only 1% in the arterial haemoglobin O2 saturation (SaO2). During control exercise (normal breathing resistance), muscle and brain DeltaHbO2 were not different from the resting levels, and only the leg muscle DeltaHb increased (4 (-2-10) microM, P < 0.05). Moderate resistive breathing increased DeltaHbO2 of the intercostal and vastus lateralis muscles to 6 +/- (-5-14) and 1 (-7-9) microM(P < 0.05), respectively, while muscle DeltaHb was not affected. Cerebral DeltaHbO2 and DeltaHb became elevated to 6 (1-15) and 1 (-1-6) microM by resistive breathing (P < 0.05). Resistive breathing caused an increased concentration of oxygenated haemoglobin in active muscles and in the brain. The results indicate that CO2 influences blood flow to active skeletal muscle although its effect appears to be smaller than for the brain.