We have studied ouabain-resistant, external sodium-stimulated, lithium efflux (Li-Na countertransport) in red blood cells from 21 borderline hypertensives with at least one hypertensive first degree relative (BH-F), 19 borderline hypertensives without family history of essential hypertension (BH-NF), and 35 age-matched normotensive subjects. The data indicate the finding of an increased Li-Na countertransport in all BH (F+NF), but with a significant overlap between BH values and control ones: Li-Na countertransport is significantly higher only in BH-F but it is normal in BH-NF. Moreover, there is a significant correlation of Li-Na countertransport to total peripheral resistance but not to mean blood pressure in all hypertensive patients. It is suggested that in BH the increase of erythrocyte Na flux is mediated by the Na-Na exchange diffusion, and its abnormality may be associated to the hereditary trait of essential hypertension rather than the high blood pressure per se, probably resulting in the development of hypertension, through the increased vascular smooth muscle tone.