Studies in our laboratory and by others have repeatedly shown that the mean value of the rate-constant for ouabain-sensitive sodium efflux is reduced and intracellular sodium increased in essential hypertension, but that there is considerable overlap with the normotensive population. There is a significant negative correlation between the rate constant and the diastolic blood pressure. The depression of the rate constant is most marked in those hypertensives with the most suppressed renin/angiotensin systems. Treatment of a population of essential hypertensives with thiazide diuretics returns the rate constant to normal. The serum of hypertensives depresses the rate constant of normal cells in vitro in proportion to the degree of abnormality present in the hypertensive's cells.