Background Because data are lacking, we examined the acute effect of exercise on ambulatory blood pressure (BP) in premenopausal white women (n = 18) and black women (n = 15) with normal (n = 21) and high (n = 12) BP. Methods Women performed 40 minutes of control and moderate-intensity exercise. BP and hormones were measured before, during, and after the control and exercise periods. By means of RMANCOVA (repeated measures analysis of covarience), we tested whether BP and hormones differed with time and between ethnic, BP, and experimental groups. Multiple regression analysis was used to examine hormonal mediators of the postexercise BP response. Results Among white women with hypertension, average daytime systolic (S) and diastolic (D) BP decreased 11.0 ± 3.3 mm Hg (−2.9, −19.1; P =.017) and 8.2 ± 2.8 mm Hg (−1.2, −13.9; P =.000), from 142.6 ± 5.8 mm Hg and 96.1 ± 2.8 mm Hg, respectively, after exercise. Among black women with high BP, mean daytime SBP rose 12.5 ± 5.2 mm Hg (−2.0, 27.1; P =.000) after exercise, from 121.8 ± 6.1 mm Hg, whereas DBP was similar before and after exercise (81.4 ± 4.3 mm Hg and 82.8 ± 4.7 mm Hg, respectively). In white women without hypertension, daytime SBP and DBP were similar before and after exercise. In black women without hypertension, mean daytime SBP increased 6.3 ± 2.6 mm Hg (0.4, 12.1; P =.000) after exercise from 103.6 ± 1.4 mm Hg, and DBP did not change. In black women, hypertension (P = 0.000) and exercise-mediated insulin decreases (P =.005) explained 85.6% of the postexercise SBP response (P =.000). In white women, hypertension (P =.003) and baseline plasma renin (P =.049) accounted for 53.3% of the postexercise SBP response (P =.001). Exercise acutely reduced daytime BP in white women, but not in black women with high BP. Conclusion Endurance exercise may adversely affect the BP of black women. (Am Heart J 2003;145:364-70.)