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OBJECTIVE: To determine whether in patients presenting with posturally related syncope administration of salt increases plasma volume and improves orthostatic tolerance. Patients with poor tolerance of orthostatic stress tend to have lower than average plasma and blood volumes. DESIGN: A double blind placebo controlled study in 20 patients and an open study in 11 of the effects of giving 120 mmol/day of sodium chloride. PATIENTS: 31 patients presenting with episodes of syncope who had no apparent cardiac or neurological disease. Plasma volume was determined by Evans blue dye dilution, orthostatic tolerance by time to presyncope in a test of combined head-up tilt and lower body suction, and baroreceptor sensitivity by the effect of neck suction on pulse interval. RESULTS: 8 weeks after treatment, 15 (70%) of the 21 patients given salt and three (30%) of the placebo group showed increases in plasma and blood volumes and in orthostatic tolerance, and decreases in baroreceptor sensitivity. Improvement was related to initial salt excretion in that patients who responded to salt had a daily excretion below 170 mmol. The patients in the placebo group who improved also showed increases in salt excretion. CONCLUSIONS: In patients with unexplained syncope who had a relatively low salt intake administration of salt increased plasma volume and orthostatic tolerance, and in the absence of contraindications, salt is suggested as a first line of treatment.