Purification of angiotensin I-converting enzyme from human lung and characteristics of the enzyme was studied. Experimental pneumonitis was produced in rabbits and the change of the activity of angiotensin I-converting enzyme was studied in purpose to clarify the role of this enzyme in the metabolism of vasoactive peptides in the lung. Purification was performed using trypsin treatment, acid treatment, DE52-cellulose column chromatography, hydroxyapatite chromatography and Sephadex G-200 gel filtration. The enzyme after final step showed a single band on disc gel electrophoresis. Experimental pneumonitis was produced by injection of Complete Freund's adjuvant (acute pneumonitis) and of N-nitroso-N-methylurethane (chronic pneumonitis). In acute experiment, angiotensin I-converting enzyme activity in pulmonary tissue and in plasma was significantly decreased. In perfusion experiment, conversion of angiotensin I to angiotensin II and inactivation of bradykinin were also significantly decreased. In case of decreased activity of angiotensin I-converting enzyme in the lung, less angiotensin II will be released into systemic circulation and bradykinin will pass through the pulmonary circulation into systemic circulation, thus this may result in the decrease of systemic blood pressure.