Abstract-The objective of this study was to determine whether there were differences in hemodynamic responses of different vascular beds to systemic administration of dipyridamole between spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. To this end, systemic hemodynamics and organ blood flows (using labeled microspheres) were determined in conscious rats before and 10 minutes after dipyridamole (4 mg ⅐ kg Ϫ1 ⅐ min Ϫ1 ) infusion. In both the normotensive and hypertensive rats, the dipyridamole infusion reduced arterial pressure by Ϸ20 mm Hg, associated with a decreased total peripheral resistance and an increased cardiac output. Renal blood flow decreased significantly in SHR after dipyridamole but remained unchanged or increased slightly in the WKY rats. There were no other differences in regional hemodynamics, including those of brain, liver, skin, and muscle, between the WKY and SHR. Antihypertensive treatment completely restored normal renal vascular response to dipyridamole. Previous reports had demonstrated an abnormal coronary hemodynamic response of the SHR. Our data demonstrate that, as with coronary hemodynamics, hypertension selectively induced alterations in renal vasculature. These findings may be of importance in identifying the earliest hemodynamic evidence of developing hypertensive nephrosclerosis. Key Words: hypertension Ⅲ renal vasculature Ⅲ dipyridamole Ⅲ regional hemodynamics S ustained increase in arterial pressure alters function as well as structure of systemic vasculature. 1-3 These hypertension-induced vascular alterations eventually lead to target organ damage of brain, heart, and kidneys. Fortunately, antihypertensive therapy seems to ameliorate these hypertensive vascular changes, reducing morbidity and mortality from strokes and coronary heart disease. 4,5 In contrast, the incidence and prevalence of end-stage hypertensive renal disease continue to increase. 6 Moreover, renal vascular involvement in hypertension is seldom detected during its early stages. At the moment when clear clinical signs of renal insufficiency become apparent, renal damage is usually so extensive that therapy can only delay, but not prevent, the development of end-stage renal failure. For this reason, early signs of renal involvement in hypertensive disease are highly desirable.In our studies dealing with the coronary circulation in spontaneously hypertensive rats (SHR), early hemodynamic alterations were demonstrated by dipyridamole infusion. [7][8][9][10] We found that coronary vasodilation in response to dipyridamole infusion was significantly diminished in SHR and that antihypertensive therapy restored vasodilatory response of coronary vasculature. This report concerns the effects of dipyridamole on the other regional circulations of normotensive Wistar-Kyoto (WKY) rats and SHR. Our purpose was to determine whether there were differences between WKY and SHR in the hemodynamic responses of various vascular beds to the systemic administration of dipyridamole, an agent commonly used ...