Abstract —The incidence of end-stage renal disease attributable to hypertension is 5-fold greater in African Americans than in whites. To determine whether glomerular hyperfiltration is an antecedent to renal failure, we compared responses of renal blood flow and glomerular filtration rate to graded infusions of norepinephrine (0.01, 0.025, and 0.05 μg · kg −1 · min −1 for 30 minutes each) in 29 African Americans and 33 age-matched French Canadian whites with essential hypertension. Renal blood flow and glomerular filtration rate were measured by using a constant-infusion technique of PAH and inulin, respectively. Studies were conducted on an inpatient clinical research center, and antihypertensive medications had been discontinued for at least 1 week. Based on 24-hour blood pressure monitoring, nighttime blood pressures decreased ( P <0.01) in the French Canadians but not in the African Americans. Baseline renal blood flow was higher ( P <0.05) in the African Americans (1310±127 mL · min −1 per 1.73 m 2 ) than in the French Canadians (1024±42 mL · min −1 per 1.73 m 2 ); baseline glomerular filtration rate was also higher ( P <0.01) in the African Americans (140±4 versus 121±4 mL · min −1 per 1.73 m 2 ). In response to norepinephrine-induced blood pressure increases, renal blood flow was autoregulated and did not change in either patient group. In the African Americans, glomerular filtration rate increased ( P <0.01) to 167 mL · min −1 per 1.73 m 2 during the first norepinephrine infusion, without subsequent change. In contrast, glomerular filtration rate did not change with norepinephrine-induced increases of blood pressure in the French Canadians. In the African Americans, the elevation of baseline glomerular filtration rate, with a further increase in response to norepinephrine, may be indicative of glomerular hyperfiltration. Glomerular hyperfiltration and lack of nocturnal blood pressure decline may contribute to the higher incidence of end-stage renal disease in hypertensive African Americans.