Previous studies from our laboratory indicate that exposure of the rat to chronic normobaric hypoxia reduces stores of active angiotensin-converting enzyme (ACE) in the lung. This study assesses directly the effects of hypoxia on ACE synthesis in cultured porcine pulmonary artery endothelial cells. Confluent cultures were exposed to hypoxia [2.5% O2 at 1 atmosphere (atm)] in a triple gas incubator; controls were cultured in normoxic conditions. After 24-, 48-, and 72-h exposure to hypoxic or normoxic conditions, followed by incubation with [35S]methionine for an additional 24 h under the same conditions, newly synthesized radiolabeled ACE was quantitated. Radiolabeled ACE was isolated by an immunobead procedure using either anti-ACE (porcine lung) immunoglobulin G (IgG) or nonimmune IgG. A single radiolabeled peak (150 kDa) with the same electrophoretic mobility as purified porcine lung ACE was observed. There was a significant time-dependent increase in endothelial cell ACE antigen synthesis without a concomitant change in either cell number or total trichloroacetic (TCA)-precipitable protein in hypoxic cells compared with normoxic controls. In contrast, ACE activity, assessed by conversion of 125I-labeled angiotensin I to 125I-labeled angiotensin II was unchanged in cultures exposed to hypoxia (2.5% O2). This suggests that an inactive form of ACE is synthesized by cultured pulmonary artery endothelial cells under hypoxic conditions.