Angiotensin converting enzyme (ACE) is stored in the endothelium. Its activity depends--among others--on the O2-concentration of the blood. Aim of the study was to examine the serum ACE values in chronic obstructive lung diseases (bronchial asthma, chronic bronchitis, lung fibrosis etc.). At the time of blood sampling, blood-gas tensions and respiratory function parameters of the patients were also determined. On the basis of the blood-gas parameters and SACE x + SD and x--SD values, obtained from the normoxic-normocapnic group, the patients could be divided into sub-groups. In contrast to data in the literature increased enzyme levels in response to hypoxia could be found only in patients suffering from a pulmonary disease associated with severe tissue damage.