We studied the effect of a change in oxygen tension on converting enzyme activity in anesthetized, paralyzed, catheterized dogs ventilated with room air, 100% O2, and hypoxic gas mixtures. Bradykinin was continuously infused into the femoral vein and simultaneous samples drawn from the pulmonary artery and left atrium; bradykinin was extracted into ethanol and measured by radioimmunoassay. Clearance of bradykinin by lung converting enzyme decreased from 96% at PaO2 levels above 95 Torr to 0% below 26 Torr. Inhibition of enzyme activity was rapid in onset (less than 2 min), closely correlated with PaO2 (r = 0.92, P less than 0.001), and reversible within 2 min after return to room air breathing. Converting enzyme activity of the systemic vascular bed was also inhibited by hypoxia; kininase I activity was unaffected by oxygen tension. Although arterial bradykinin concentrations in the range of 0.5 ng/ml produced hypotension in normoxic animals, elevations to 30 ng/ml had no hypotensive effect in hypoxic dogs. During acute hypoxia, venous bradykinin will pass through the lung unmetabolized, and local levels of angiotensin II and bradykinin will vary in vascular beds with different oxygen tensions, providing a finely-graded mechanism for blood flow regulation.