In eleven hypertensive patients with renal artery stenosis, the acute renal effects of captopril were investigated using two methods: 1) the Tc99m-DTPA renography with determination of an index of renal perfusion (IP), an index of glomerular filtration (IF) and the ratio of these indices (F/P); 2) the renal hemodynamics obtained by the clearance method using a continuous infusion of I131-hippuran and I125-iothalamate for calculation of renal blood flow (RBF), glomerular filtration rate (GFR) and filtration fraction (FF). The studies were performed before and after captopril treatment. Patients were classified according to the acute response to captopril as responders (R; n = 6) and non responders (NR; n = 5). Results are as follows: (B: basal, C = captopril). (table; see text) These data confirm that captopril produced a significant decrease in F/P and FF in R whereas these indices did not change in NR; it was found that IF and GFR decreased in R whereas IF increased and GFR did not change in NR; a significant correlation was observed between delta IF and delta GFR in R (r = 0.834, p less than 0.05). These results indicate that 1) data obtained before and after captopril by renography and by clearance methods are in good correlation either in Ror in NR patients; 2) Captopril test including renography or renal hemodynamic measurements is useful for selection of R patients.