We determined the effect of chronic administration of the angiotensin converting enzyme (ACE) inhibitor, enalapril, on the in vivo pulmonary inactivation of bradykinin (BK) and conversion of angiotensin I (Ang I). In addition we assessed whether chronic ACE inhibition influenced the activity of prolylendopeptidase (PEP), which metabolizes Ang I to generate angiotensin-(1-7) (Ang-[1-7]) and inactivates BK. Male Wistar rats were treated orally with enalapril (10 mg/kg once a day) for 7 to 15 days (n = 20) and 21 to 30 days (n = 11). Vehicle-treated rats (7 to 30 days, n = 11) were used as controls. Pulmonary inactivation of BK and conversion of Ang I were determined in conscious enalapril- or vehicle-treated rats before and after intravenous administration of the ACE inhibitor enalaprilat (MK-422, 10 mg/kg). Pulmonary inactivation of BK (%) was determined by comparing equipotent doses of BK injected by the intravenous and intraaortic routes, and Ang I conversion (%) by comparing the pressor effect of Ang I and Ang II injected intravenously. PEP-like activity in plasma and lung homogenates was determined fluorometrically using the synthetic substrate Suc-Gly-Pro-MCA. In control rats, pulmonary BK inactivation averaged 97.6% +/-0.54%. Acute ACE inhibition with MK-422 reduced BK inactivation to 42.0% +/- 2.7%. However, in rats treated chronically with enalapril, BK inactivation was increased as compared with acute ACE inhibition, averaging 58.8% +/- 3.7% at 7 to 15 days and 58.8% +/- 4.5% at 21 to 30 days of treatment. Intravenous administration of MK-422 to the enalapril-treated rats did not return the increased BK inactivation to the level observed during acute ACE inhibition. In contrast, Ang I conversion was significantly reduced from 46.7% +/- 6.5% to 0.9% +/-0.2% by MK-422, and this inhibition remained essentially unchanged during chronic treatment. PEP-like activity in plasma and lung homogenates of control rats was 4.4 +/- 0.3 nmol MCA/min/mL and 11.4 +/- 0.9 nmol MCA/min/mg protein, respectively. After chronic treatment with enalapril there was a progressive increase of PEP-like activity in both plasma and lung, which after 21 to 30 days of treatment averaged 10.7 +/- 1.7 nmol MCA/min/mL and 29.2 +/- 2.8 nmol MCA/min/mg protein, respectively. These data indicate that chronic ACE blockade induces alternative BK-inactivating mechanisms and increases Ang-(1-7)-generating mechanisms.