The overall objectives of this study are to examine the relationships between circulating vitamin D, insulin sensitivity, and multiple indices of vascular function and to examine whether vitamin D deficiency in African Americans (AA) and White Hispanics (WH) is responsible for ethnic differences in insulin sensitivity and hypertension in AA, WH and European Americans (EA), as well as mechanisms underlying the association between insulin resistance and blood pressure. We hypothesize that 1) serum 25(OH)D is associated with insulin sensitivity and vascular functioning, independent of adiposity, 2) lower insulin sensitivity and vascular functioning in AA and WH relative to EA is due to lower circulating 25(OH)D in AA, and 3) the relationship between insulin resistance and vascular dysfunction is mediated by 25(OH)D.