Eight normotensive obese subjects participated in an inpatient study designed to determine the effect of a constant sodium intake (150 mEq) on the renin-aldosterone axis during 12 weeks of weight reduction. Two 800-calorie (3,200 kj) ketogenic diets, differing in carbohydrate content (10 g vs 70 g) were used for the study. Supine and upright plasma renin activity (PRA) and serum aldosterone (SA) were determined at the baseline and every 4 weeks. Total body water (TBW) was determined by the tritiated water technique at the baseline and 12 weeks after dieting. Extracellular water (ECW) was determined by 77Br space. Routine serum chemistries were obtained at 2-week intervals. Analysis of variance indicated no significant differences in the PRA and SA between the two diets. At the baseline, while on a self-selected 150 mEq sodium diet, there was a 3- to 4-fold increase in PRA after 2 h of ambulation (supine PRA 0.83 +/−0.22 increased to 3.41 +/−0.96 ng/ml/h). After the hypocaloric diets were instituted, the absolute values for PRA in the supine and upright positions declined. However, the magnitude of the postural response (3- to 4-fold increase) remained unchanged during the 12 weeks of weight reduction. There was no decline in the absolute values for supine or upright SA, during the entire study. Weight loss was significant (from 102.56 +/−6.0 to 81.7 +/−3.7 kg; P less than .001) and was accompanied by a mean +/−SE reduction in the TBW of 3.01 +/−0.88 liters (P less than .011).(ABSTRACT TRUNCATED AT 250 WORDS)