This report deals with the possibility that there is a specific change of the lithium transport across the membrane of erythrocytes from patients with essential hypertension. Sodium-lithium countertransport was significantly increased (p less than 0.005) in erythrocytes from 17 males with essential hypertension (mean 0.7 mmole (liter cells X hr)-1, range 0.4-1.6) compared to a group of 16 normotensive males (mean, 0.4 mmole (liter cells X hr)-1, range 0.3-0.6). A considerable overlap between the values from patients and controls was found. No significant increase of the transport function was found in a group of 14 female patients (mean 0.4 mmole (liter cells X hr)-1, range 0.2-0.6) compared with 10 normotensive female controls (mean 0.3 mmole (liter c hr)-1, range 0.3-0.6). A considerable overlap between the values from patients and controls was found. No significant increase of the transport function was found in a group of 14 female patients (mean 0.4 mmole (liter cells X hr)-1, range 0.2-0.6) compared with 10 normotensive female controls (mean 0.3 mmole (liter c hr)-1, range 0.3-0.6). A considerable overlap between the values from patients and controls was found. No significant increase of the transport function was found in a group of 14 female patients (mean 0.4 mmole (liter cells X hr)-1, range 0.2-0.6) compared with 10 normotensive female controls (mean 0.3 mmole (liter cells X hr)-1, range 0.1-0.6). Determination of sodium-lithium countertransport in red blood cells from nine children with and 14 without known familial disposition for essential hypertension did not demonstrate a close coupling between genetic disposition and the membrane transport function. In spite of the very small intraindividual variability of the transport function, studies of changes in sodium-lithium counter-transport are hampered by considerable interindividual variability of the transport in red cells from apparently normal individuals.