On the isolated perfused rat kidney, the angiotensin converting enzyme (ACE) activity was evaluated with two approaches: one, pharmacological, through the vasoconstrictor response to angiotensin I (ANG I), and the other, biochemical, through the measurements of the enzymatic activity on renal homogenate. ANG I and angiotensin II (ANG II) induced a concentration-dependent renal vasoconstriction(EC50 = 10.5 ± 1.8 × 10−9and 1.1 ± 0.5 × 10 −9 M, respectively). Both responses were competitively antagonized by an ANG II receptor antagonist, saralasin (pA2 = 8.65 ± 0.63 and 8.94 ± 0.28, respectively). The effects of ACE inhibitors were studied in vitro after addition of captopril and ramiprilat (10−5M) directly to the perfusion medium, and ex vivo, after pretreatment of the rats with ramipril (50 mg/kg, i.p. the day before or 10 mg/kg/day, per os, over 3 weeks). In spite of the high doses of ACE inhibitors used, the ANG I concentration-response curve was only shifted to the right by a factor of 4, although renal tissue ACE activity was completely inhibited. Saralasin (10−5M) totally abolished the ANG I-induced vasoconstriction elicited in the presence of ACE inhibitors, this response being therefore linked to a generation of ANG II from ANG I. Our results suggest that, on the isolated perfused rat kidney, ANG II may be formed from ANG I by a peptidyl dipeptidase different from the ACE.