The hydrolytic cleavage of angiotensin I has been studied in homogenate preparations of rat lung and aorta using gradient elution HPLC to monitor the formation of peptide products. Fresh crude homogenate preparations produced a rapid breakdown of angiotensin I to largely unidentifiable fragment peptides. Neither His-Leu nor angiotensin II was observed in these preparations even in the presence of captopril (20 microM) and the amino-peptidase inhibitors, puromycin, amastatin and bestatin. However, in freeze-thawed homogenates, angiotensin II and His-Leu were detectable together with the tetrapeptide, angiotensin (1-4). The addition of captopril (20 microM) reduced the amount of angiotensin II produced but did not completely block its formation. Higher concentrations of captopril or the addition of enalaprilat or EDTA did not further reduce the amount of angiotensin II produced. In the presence of captopril a peptide corresponding to des-Leu(10)angiotensin I was formed in relatively large amounts (equivalent to 40% of angiotensin I catabolized). Homogenates purified by concanavalin A affinity chromatography gave a clean hydrolysis of angiotensin I to angiotensin II and His-Leu which was completely blocked by captopril. These results suggest an ACE-like activity in rat lung and aorta that is not sensitive to converting enzyme inhibitors.