Evidence supports the hypothesis that hyperinsulinemia, especially in obesity, contributes to salt-sensitive hypertension by enhancing sodium retention and blunting the normal reduction of sympathetic drive and vascular resistance that occurs during a high versus low NaCl diet. To address these issues, we studied 18 obese (body mass index, > 27 kg/m2) subjects younger than 45 years old with mild hypertension to determine if the salt-sensitive versus salt-resistant subset had higher insulin levels, retained more volume, and failed to suppress sympathetic drive and vascular tone normally on a high (approximately 200 mEq/d) versus low (20 mEq/d) NaCl diet for 7 days each. Six obese subjects were salt sensitive, with an 8.4 +/- 2.1 (SEM) mm Hg increase of ambulatory mean blood pressure on the high versus low NaCl diet. Ten obese subjects were salt resistant, with a 7.1 +/- 0.9 mm Hg reduction of ambulatory mean blood pressure on high versus low NaCl. The salt-sensitive and salt-resistant groups had similar values, respectively, for the insulin area under the curve during an oral glucose tolerance test on low (14.6 +/- 1.8 versus 14.0 +/- 1.4 mU x min/dL, P = NS) and high (10.6 +/- 1.5 versus 10.6 +/- 1.0, P = NS) salt diets. Although insulin levels were similar, insulin raised calf blood flow in salt-resistant subjects (P < .05) but not in salt-sensitive subjects on the high NaCl diet.(ABSTRACT TRUNCATED AT 250 WORDS)