In 83 patients with severe, largely irreversible bronchial obstruction (FEV1/VC less than 40% and FEV1 after orciprenaline inhalation less than 120% of the control value) and radiologie evidence of AD-emphysema, arterial PCO2 and PO2 were measured at rest in supine position and on the bicycle ergometer during a steady-state exercice of 5 min. Alveolar hypoventilation (PCO2 greater than 45 mm Hg) was most often observed in the cases with FEV1 less than 1.01 (in 22 patients [27%] at rest and in 26 patients [31%] during exercise). However, there was no significant correlation of the PCO2 increase with the degree of bronchial obstruction. In all patients there was a marked inhomogeneity of the alveolar ventilation or the alveolar-capillary O2 transfer in relation to alveolar blood perfusion. Indeed, the alveolar-arterial PO2 difference was increased (40 mm Hg at rest and 45 mm Hg during exercise). The additional increase of this gradient during exercise was due to an unequal distribution of alveolar O2 diffusing capacities in connection with the alveolar blood flow (preceding measurements of the N2 gradient between alveolar air and arterial blood revealed a mean fall from 18 at rest to 8 mm Hg during exercise). The observed deterioration of hypoxemia during exercise (without additional hypercapnia) is to a great extent related to the degree of bronchial obstruction measured by simple spirometry. Thus, measurements of PCO2 and PO2 at rest and during exercise appear to be a helpful adjuvant to routine spirometry in the diagnosis of subclinical emphysema.