Sodium efflux rates were measured in leucocytes from eighteen normotensive subjects who had one or more first-degree relatives with essential hypertension and from twenty-four matched controls with no such family history. The total efflux rate constant was significantly lower in those with a family history of hypertension, owing to reduced ouabain-sensitive sodium pump activity. The presence of a membrane electrolyte handling abnormality characteristic of essential hypertension in normotensive individuals genetically predisposed to hypertension points to an underlying genetic factor. At the same time, the fact that blood-pressure was normal in these subjects indicates that the abnormality does not participate directly in blood-pressure elevation. Rather, the abnormality, like other red-cell changes in electrolyte handling, seems to be a marker for a genetically determined alteration in membrane structure, and thus only indirectly related to hypertension.