The present study was performed to determine the role of afferent arterioles in the impaired autoregulatory response shown to occur In the contralateral kidney of Goldblatt hypertensive rats. The responsiveness of juxtamedullary afferent arterioles to alterations in perfusion pressure was studied in the nonclipped kidney of two-kidney, one clip hypertensive and sham-operated rats. Systolic pressure, 5-6 weeks after clipping, averaged 184±6 mm Hg in the hypertensive rats (n = 16) and 121 ±3 mm Hgin the sham-operated control rats (n=7). By using the in vitro blood-perfused juxtamedullary nephron technique, afferent arterioles were directly visualized, and their inside diameters were measured by videomicroscopic methods. In sham-operated kidneys perfused with blood from normotensive rats, afferent arteriolar diameter averaged 22.8±1.8 μm at a renal arterial perfusion pressure of 151 ± 1 mm Hg and increased to 24.8±1.8 μm when perfusion pressure was reduced to 110±2 mm Hg. Conversely, in hypertensive kidneys perfused with blood from either hypertensive or normotensive rats, the afferent arterioles failed to vasodilate and actually exhibited a slight decrease in diameter from 24.6±1.3 to 23.0±2.3 μm in response to the same reduction in perfusion pressure. Vasodilator capability, however, could be demonstrated in response to verapamil and sodium nitroprusside, which increased afferent diameter in both the sham-operated and hypertensive groups of rats. Thus, unlike arterioles from normotensive rats, juxtamedullary afferent arterioles from two-kidney, one clip Goldblatt hypertensive rats fail to vasodilate after a reduction in perfusion pressure. This impaired autoregulatoty ability might result from an intrinsic functional defect in the vasculature of the high pressure contralateral kidney. © 1990 American Heart Association, Inc.