To examine the relationship between diet, blood pressure, and plasma insulin concentrations, we studied 14 healthy males who were prescribed low-fat and high-fat diets. The low-fat diet contained 25% (of energy intake) fat and 54% carbohydrate; the high-fat diet was 45% fat (predominantly saturated fat) and 36% carbohydrate. The diets were consumed over consecutive 2-week periods in random sequence, separated by a 2-week washout period. Resting supine systolic and diastolic blood pressures decreased significantly by 7 and 3 mm Hg, respectively, and plasma total cholesterol, LDL cholesterol, and HDL cholesterol concentrations all fell (by 21.6%, 25.7%, and 18.0%, respectively; allP<0.001) on the low-fat compared with the high-fat diet. Fasting glucose and the glucose area under the curve during the frequently sampled intravenous glucose tolerance test (300 mg/kg glucose load with blood sampling for 180 minutes) were significantly lower, and the glucose disappearance rate tended to be faster after the low-fat diet. In contrast, fasting insulin concentrations and the insulin response (insulin area under the curve) to glucose challenge were unchanged. Insulin sensitivity (defined as the rate of glucose disappearance per unit of insulin increase during the period 0 to 40 minutes after the glucose load) was significantly higher on the low-fat diet. These results suggest that the hypotensive effects of a low-fat, high-carbohydrate diet, although associated with an improvement in insulin sensitivity, are not mediated by changes in plasma insulin concentration.