We tested whether sodium restriction would counteract the decrease in sympathetic nervous system activity usually associated with marked energy restriction. The effects of two levels of energy restriction, with different sodium intakes, on plasma norepinephrine (NE) levels while supine and in response to standing were studied. Twenty-two healthy normotensive obese female subjects (body mass index, 34 +/- 1 kg/m2; weight, 90 +/- 2 kg) followed one of three 3-week protocols: 1) total fasting with 80 mmol/day NaCl, 2) a very low energy diet (VLED) containing 1.7 MJ, 93 g protein, and 90 mmol Na/day, with an additional 60 mmol/day NaCl supplement, or 3) total fasting without NaCl (0 Na fast). At the end of the baseline isocaloric diet and of total fasts or VLED, pulse, blood pressure, and plasma NE were measured after 4 h of recumbency and 5 and 10 min after assuming the upright posture. These measurements were repeated after 1 L physiological saline was infused into the 0 Na fast subjects. Cumulative negative sodium balance was observed only in the 0 Na fasting subjects. Supine blood pressure decreased from baseline with fasting, but not with the VLED. The decreases in systolic pressure and increases in heart rate on standing observed with all diets were greatest with the 0 Na fast. Supine plasma NE (vs. baseline value) declined (P less than 0.05) with the VLED, remained unchanged with the Na supplemented fast, but increased with the 0 Na fast (P less than 0.05). The upright plasma NE values were highest in the 0 Na fast subjects, but lower after the saline infusion as well as in the subjects on the VLED. Thus, the decrease in NE due to energy restriction with normal sodium intake was counteracted by moderate sodium restriction, and levels increased with zero sodium intake. Therefore, sodium depletion can override the suppressive effect of energy restriction and, instead, increase the activity of the sympathetic nervous system, as reflected by plasma NE.