Ultrasonic duplex scanning has been validated as a noninvasive method to evaluate the kidney arteries and hemodynamic characteristics of renal blood flow in patients with renal artery stenosis. The purpose of our study was to assess the changes in renal vascular impedance in 22 patients with renovascular hypertension, as compared with 45 essential hypertensives and 15 normotensives, by using the Doppler parameter resistance index (RI) before and after a captopril oral test. After the captopril test the delta RI decreased significantly in the stenotic artery (P < 0.05). Univariate analysis showed that PRA values after captopril correlated inversely with the changes of RI only in the stenotic artery (P < 0.05). Thus, our findings suggest that the application of the captopril test to renal echo-Doppler may represent a feasible, noninvasive, and inexpensively useful tool in the screening studies aimed at diagnosing renovascular hypertension.