Several laboratories have examined the relation between salt intake and blood pressure in both experimental animals and humans. The human studies have used widely varying methodologies and different criteria for sodium sensitivity. Nonetheless, these studies have produced convincing data that the blood pressure of some individuals is far more sensitive to the effects of sodium depletion or loading than that of others. Furthermore, a minority of the population appears to comprise acutely salt-sensitive individuals. Some studies have shown that sodium-sensitive individuals share several characteristics. They include increased forearm vascular resistance, decreased venous compliance, suppressed plasma renin activity, and lower circulating aldosterone concentration. These findings have also been described in the Dahl salt-sensitive rat, which suggest a genetic link in humans as well as the rat. Long-term follow-up of sodium-sensitive and sodium-resistant groups has shown that although blood pressure levels are approximately equal in the two groups during sodium depletion, resumption of a daily sodium intake of about 150 meq results in significantly higher levels of blood pressure and forearm vascular resistance in the sodium-sensitive group. This difference persists for at least 12 months.