45 patients with chronic obstructive lung disease and chronic respiratory insufficiency were studied. The investigation of these hypoxemic patients concerned the compensating mechanisms in the oxygen transport. The study was performed at rest, during exercise and after 100% oxygen breathing. Normal values for the calculated oxygen consumption (VO2) and cardiac output (Q) were found. Low values of arterial oxygen pressure (PaO2) were compensated by a decrease of the venous oxygen pressure (PvO2) normalizing the arteriovenous oxygen saturation difference (Sa-v)· In patients with very low values of PaO2 a further decrease of PvO2 was not established. In these patients, Sa-v became smaller and an increase of the hemoglobin concentration resulted as a compensating mechanism. During exercise, a shift to the right of the oxygen dissociation curve took place as an added compensation