The angiotensin I converting enzyme has two important functions: it inactivates bradykinin and converts angiotensin I to angiotensin II. Inhibition of the enzyme blocks the renin-angiotensin system and decreases systemic blood pressure if the pressure is maintained or increased by renin. The enzyme occurs in a variety of tissues and cell forms. The vascular endothelial cells of the lung and of peripheral blood vessels, and the epithelial cells of the kidney tubules are major sources of the enzyme. In addition to inactivating hypotensive peptides and activating a hypertensive one in the systemic circulation, the enzyme may affect organ functions by hydrolyzing peptides that are formed and released locally.