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We studied the effects of atrial natriuretic factor (ANF) on the conversion of angiotensin I to angiotensin II. Pulmonary artery endothelial cells converted 1.22nmol/min per dish [125I]angiotensin I to angiotensin II, but ANF suppressed the conversion by 0.475 nmol/min per dish. The maximum rate of inhibition of angiotensin converting enzyme (ACE) activity by ANF was 60% at 10−6mol/l ANF compared with control conditions. The amino acid fragments of ANF were studied to determine whether its specific structure was necessary for inhibiting ACE in endothelial cells. We found that atriopeptin II (fragment 5–27) inhibited ACE activity as much as ANF. Fragment 7–25 had the same rate of inhibition, but fragment 13–27 had no effect on the conversion of angiotensin I to angiotensin II. These results suggest that the amino acid sequence of Cys (7)–Cys (23) in ANF is important for the inhibition of ACE in pulmonary endothelial cells.