Erythrocyte Na/K flux ratio was examined in relation to a family history of essential hypertension (FH-HT) and plasma and urine electrolytes in 84 normotensive children (13-15 yrs old), and in relation to sodium intake in six children with acute glomerulonephritis or IgA nephropathy who had normal renal function (6-13 yrs old). Erythrocyte Na/K flux ratio was significantly lower in children with a family history of essential hypertension than in those without. Plasma and urine electrolytes (Na and K) showed no significant differences between children with and without a family history of essential hypertension, although erythrocyte Na/K flux ratio was negatively correlated with serum K level in the whole group. Furthermore, erythrocyte Na/K flux ratio was unchanged before and after the restriction of sodium intake in children with nephropathy. These findings suggest that the erythrocyte Na/K flux ratio may be suppressed in normotensive children with a family history of essential hypertension as previously reported in hypertensive adults, and that plasma K level should be considered first when evaluating the erythrocyte Na/K flux ratio.