This study examined whether the presence of hypertension, an insulin-resistant condition, exacerbates the defect in insulin action observed in obesity and type II diabetes mellitus. Glucose metabolism in the basal state and in response to insulin was quantitated by using the euglycemic insulin (20 mU.min-1 x m-2) clamp in combination with 3-[3H]glucose infusion and indirect calorimetry in 20 obese nondiabetic subjects (10 hypertensive and 10 normotensive), 26 type II diabetic subjects (13 hypertensive and 13 normotensive), and 11 normal nondiabetic subjects. The two groups of obese subjects and the two groups of diabetic subjects were matched for sex, age, race, body mass index, and fat distribution. Both in the basal state and during insulin infusion, glucose disposal rates (total, oxidative, and nonoxidative) were similar in obese subjects with or without hypertension. Compared with control subjects, both groups of obese subjects were markedly insulin resistant. Similarly, type II diabetic individuals, whether normotensive or hypertensive, were equally insulin resistant. The severity of insulin resistance was nearly identical in obese and diabetic groups. In diabetic subjects, the inhibitory effect of insulin on hepatic glucose output, lipolysis, and lipid oxidation was blunted compared with normal subjects. In obese subjects the ability of insulin to inhibit lipolysis and lipid oxidation was impaired. However, hypertension did not alter the suppressive effects of insulin on hepatic glucose production, plasma free fatty acid levels, or lipid oxidation in either obese or type II diabetic subjects. These results indicate that hypertension does not confer a greater severity of insulin resistance than that already is present in obesity and type II diabetes mellitus.