Angiotensin (Ang) receptor blockers (ARBs) increase bradykinin (BK) by antagonizing Ang II at its type 1 (AT(1)) receptors and diverting Ang II to its counterregulatory type 2 (AT(2)) receptors. Because the effect of ARBs on BK is constrained by the short half-life of BK and because ACE inhibitors block the degradation of BK, this study was designed to test the hypothesis that an ACE inhibitor can potentiate ARB-induced increases in renal interstitial fluid (RIF) BK levels. We used a microdialysis technique to recover BK and cGMP in vivo from the RIF of sodium-depleted, conscious Sprague-Dawley rats infused for 60 minutes with the AT(1) receptor blocker valsartan (0.17 mg/kg per minute), with the active metabolite of the ACE inhibitor benazepril (benazeprilate, 0.05 mg/kg per minute), or with the specific AT(2) receptor blocker PD 123,319 (50 microg/kg per minute) alone or combined. Each animal served as its own control. RIF BK and cGMP levels increased significantly over 1 hour in response to valsartan, benazeprilate, or both but not to a vehicle control (P<0.01). The combined benazeprilate-valsartan effect was greater than the sum of their individual effects, suggesting potentiation rather than addition, and was abolished by PD 123,319. We demonstrate for the first time that an ACE inhibitor (benazepril) and an ARB (valsartan) potentiate each other, and we postulate that such combinations may be beneficial in clinical states marked by Ang II elevation, such as chronic heart failure, postinfarction left ventricular dysfunction, and hypertension.