Obesity is a risk factor for arterial hypertension. We studied the relationships between the body mass index (BMI) and the nycthemeral pattern of blood pressure (BP), renal function and sodium and water excretion (EX) in a group of 25 moderately hypertensive untreated men (41 +/- 2 y, 80 +/- 3 kg). Subjects were given a high sodium diet (6 g NaCl added to their usual diet, daily EX=200 mmol). On the 7th day, BP was monitored during 24 h and urine collected in 2 fractions (day=D, 8:00-22:00 and night=N, 22:00-8:00). Subjects were a posteriori divided into 2 groups according to the median BMI (26 kg/m2): Group 1, n=12, BMI 23.2 +/- 0.6 (mean +/- SEM) and Group 2, n=13, BMI 29.2 +/- 0.5 kg/m2. No difference was observed between the two groups for age, 24 h urine and sodium EX, or systolic and diastolic BP. However, heart rate was significantly higher during N in Group 2 (66 +/- 2 vs 57 +/- 2 b/min, p=0.012). Na and water EX were significantly higher during D than during N in Group 1, but lower during D than during N in Group 2. Creatinine clearance was higher in Group 2 than in Group 1 especially during N (D+29%, p=0.013; N+49%, p<0.001). In Group 2, subjects concentrated their urine more than in Group 1, as evaluated from the urine/plasma creatinine ratio (+49%, p=0.019). This ratio was positively correlated to BMI during D (r=0.561, p=0.004) but not during N. These results show that the glomerular hyperfiltration associated with overweight is more intense at night and that moderately overweight hypertensives have a reduced sodium and water EX during the day and a compensatory larger EX at night. The reduced diurnal EX goes along with an increased urine concentration. The nocturnal rise in EX is concomittant with a rise in heart rate. Given the growing health problems linked to obesity and hypertension, these results open a new field for the understanding of the difficulty to excrete sodium in this condition.