The effects of an angiotensin converting enzyme inhibitor, SA-446, on regional hemodynamics were examined in conscious renal and spontaneously hypertensive rats (RHR, SHR) using the radioactive microsphere method. We observed the differences in regional hemodynamics between hypertensive rats and normotensive rats, as well as the differences between SHR and RHR. An i.v. administration of SA-446 at a dose of 1 mg/kg induced a considerable hypotension in both types of hypertensive rats and increased the flow rates in most organs, along with an increase of cardiac output. The systemic and regional hemodynamic changes due to SA-446 in SHR and RHR were not qualitatively different, but the organs in RHR responded to SA-446 with greater magnitude. SA-446 increased the flow rates in organs which originally had low flow rates, more than it increased those of normotensive control rats.