This study had three main objectives: to examine in patients presenting with unexplained syncope the relationship between orthostatic tolerance and dietary salt intake; to examine in patients with relative low baseline salt excretion the effect of salt loading on both orthostatic tolerance and blood pressure; and to examine the relationship between dietary salt intake and the sensitivity of the baroreceptor reflex. In 178 patients with unexplained syncope we determined 24-hour urinary sodium excretion, supine arterial blood pressure, carotid cardiac baroreceptor sensitivity (neck suction) and tolerance to orthostatic stress (head-up tilt and lower body suction). Those with low salt excretions and poor orthostatic tolerance were given salt supplements and reassessed after three months. Baseline studies revealed that patients with sodium excretions < 170 mmol/day had significantly lower orthostatic tolerance than those with higher excretions. Salt loading caused a small but significant increase in mean pressure and significant increases in orthostatic tolerance and baroreceptor sensitivity. Improved orthostatic tolerance was seen in 68 of 98 (69 %) patients in whom salt was given. These patients had significantly greater changes in baroreceptor sensitivity than those failing to improve. These results confirm the benefits of salt loading patients with posturally-related syncope. Incidence of hypertension following salt in these patients is relatively uncommon but it is advisable to monitor the effects on blood pressure.