In 10 exercising patients with chronic obstructive lung disease (COLD) we measured ventilation (VE), end-tidal CO2 (PETCO2), mean inspiratory flow (VT/TI), the ratio of inspiratory time to total time for one cycle (TI/TTOT), and occlusion pressure at 0.1 s measured at the mouth (P O.1), when they breathed room air and 100% oxygen. Oxygen breathing increased the maximal work load achieved. Furthermore, at the same exercise load, P O.1, VT/TI, VE, heart rate, respiratory frequency (f) decreased significantly in hyperoxia as compared with normoxia. Thoraco-pulmonary impedance assessed by P 0.1/VE and P 0.1/(VT/TI) ratios showed a slight but significant decrease to the respiratory centre. Thus in exercising patients affected by COLD, hyperoxia not only decreases the chemical afferent drive but also may slightly reduce the afferent mechanical drive to the respiratory centre. The decrease in the thoraco-pulmonary impedance may be explained either by an increase in efficiency of the respiratory muscles and/or a decrease in airway resistance.