Two-kidney, one-clip Goldblatt models were created in Wistar rats by placing a stenotic clip around the left renal artery. A converting enzyme inhibitor, captopril was started just after operation (group 1), 4 weeks (group 2) or 12 weeks after operation (group 3). The plasma and renal renin activity (PRA and RRA) were measured by radioimmunoassay. Group 1 rats, being normotensive during captopril treatment, developed hypertension after cessation of captopril. The PRA was normal, while the RRA of ischemic kidneys was higher than that of the opposite kidneys. In group 3 rats captopril corrected hypertension in 4 of 6 rats. Although after 3 months their PRA was elevated, no difference was found in the renin content of kidneys. Group 2 rats sustained hypertension despite continued captopril treatment. These data suggest that in the early and chronic stage of this type of renovascular hypertension the renin-angiotensin system may play a major role in initiating and maintaining hypertension. In the intermediate stage (group 2), another mechanism may be involved since captopril had no effect while the pattern of the PRA and RRA of group 2 was almost the same as that of group 1.