Serum angiotensin converting enzyme activity (ACE), plasma renin activity (PRA), blood pressure (BP), blood pH, blood gases and lung function parameters were measured in patients with emphysema, extrinsic and intrinsic asthma, malignant pulmonary neoplasms, active sarcoidosis and healthy adults. Serum ACE activity was significantly higher in sarcoidosis (250.22+/-34.18 U/L); in small cell carcinoma of lung (155.10+/-38.25 U/L); emphysema (149.82+/-18.31 U/L); extrinsic asthma (141.22+/-25.30 U/L) and lower in intrinsic asthma (98.12+/-15.11 U/L) and squamous cell carcinoma of lung (97.294+/-18.85 U/L) when compared with that of control subjects (108.20+/-13.15 U/L). PRA and BP values of the patients with sarcoidosis, emphysema and small cell carcinoma were markedly elevated and sACE activity was found to be correlated with PRA and mean BP in the same diagnostic groups. sACE activity, PRA and BP of smokers were higher than those of non-smokers in control subjects and in patients with emphysema, extrinsic asthma and small cell carcinoma of lung. Oxygen tensions of the patients with emphysema , extrinsic asthma and small cell carcinoma of lung were found to be significantly decreased. Negative correlations between the sACE activity and oxygen tension (r= -0.68) and between the sACE activity and lung function parameters (r= -0.69 ) were found in these diagnostic groups suggesting that increased sACE level might appeared as a response to chronic hypoxia in the patients with emphysema, extrinsic asthma and small cell carcinoma of lung.