Numerous laboratories have confirmed that the maximal rate of sodium-lithium countertransport is increased in red blood cells from individuals with essential hypertension. The sodium-lithium countertransport is normal in secondary forms of hypertension and increased levels in essential hypertension cannot be attributed to age, body size, gender or dietary salt intake. The highest levels are observed in individuals with a positive family history of essential hypertension and the lowest levels occur in normotensive controls without hypertensive relatives.