We sought to examine the association of insulin, insulin sensitivity, and blood pressure using ambulatory blood pressure monitor in three ethnically distinct populations. The study population comprised the following. Group 1 (n = 31): African-Americans; Group 2 (n = 27): recent African immigrants; and Group 3 (n = 31): white Americans who were residing in Franklin County, Ohio. Quantitative insulin sensitivity index (Si) was obtained using the minimal model method in both groups of African ancestry and white Americans. The mean insulin sensitivity index (Si) was similar in the African-Americans (3.23 +/- 0.47 x (10)-4 x [min-l x microU/mL (-1)])(-l) and African immigrants (2.53 +/- 0.27). However, these Si values were significantly (P < .01) lower in people of African ancestry than in white Americans (6.56 +/- 1.07). The mean systolic (SBP) and diastolic blood pressure (DBP) and heart rates (HR) during 24-h and daytime periods were not significantly different in the African-Americans and African immigrants. During the night, whereas the mean SBP was not different in the three groups, DBP and HR were significantly (P < .05) higher in both groups of African ancestry than in white Americans. However, we found no significant relationships among serum insulin levels, insulin sensitivity, and ambulatory blood pressure (systolic and diastolic and mean arterial pressure) and heart rates in any of the groups. In summary, our present study demonstrates that people of African ancestry manifest 1) significantly lower insulin sensitivity indices and 2) blunted physiologic reduction in nocturnal DBP and HR when compared to white Americans who reside in the same environment. We speculate that these chronobiological alterations in BP and HR in blacks appear to be genetically determined.