Association between obesity and hypertension has been well recognized. A reduction in the body weight of over-weight hypertensive patients is a recommended lifestyle modification. The purpose of our study is to examine the relationship of insulin sensitivity and autonomic nervous activity with reduction of blood pressure by the calorie restriction. We evaluated the heart rate variability, nocturnal change of blood pressure and insulin resistance before and after a short-term low-calorie diet in 12 overweight essential hypertensives. After a week of standard diet (2000 kcal), 2 weeks of low-calorie diet (800 kcal) with normal sodium content induced a significant reduction in body mass index, triglyceride, fasting immunoreactive protein, homeostasis model assessment as an index of insulin resistance, and urinary excretion of sodium and potassium. Systolic blood pressure was significantly reduced both in daytime and night-time after the low-calorie diet (daytime, 134.5+/-6.0 to 122.0+/-4.1 mmHg; night-time, 126.8+/-5.2 to 113.4+/-7.2 mmHg). In daytime, diastolic blood pressure was also reduced (90.3+/-2.1 to 88.1+/-4.8 mmHg). Although heart rate was not significantly reduced, a rise of high frequency in night-time (346+/-82 to 572+/-108 ms2) and a fall of low frequency/high frequency in day-time (3.5+/-0.4 to 2.6+/-0.1) was significant after a low-calorie diet. In conclusion, weight loss by low-calorie diet with a constant intake of sodium, reduced blood pressure in obese hypertensives by improvement of vagal nervous activity and insulin resistance.