Although inappropriately low oxygen delivery, due to exercise-induced right ventricular dysfunction, impaired left ventricular function and/or oxygen desaturation, may have modified the increase in arterial lactic acid in patients, we do not think that these factors played a predominant role. The end-exercise oxygen saturation was similar among the different classes of patients, while the increase in lactate differed markedly. The absence of correlation between oxygen saturation and plasma lactate concentration has also been noted by several authors . Previous reports have indicated that the increase in cardiac output during exercise is normal in the majority of COPD patients with similar airflow obstruction as those of the present study . Furthermore, a recent study conducted in our laboratory showed that the increase in leg blood flow during exercise in COPD was comparable with that of age-matched normal subjects and that there was no relationship between blood lactate concentration and peripheral oxygen delivery (unpublished data). Finally, lactate production by the respiratory muscles or reduction in lactate clearance by the liver are unlikely to have contributed significantly to the abnormal lactate kinetics found in patients .