Erythrocyte Na/K flux ratio, serum sodium (Na) and potassium (K), and urinary excretions of Na and K were measured in 81 normal children on a normal diet, aged 13 to 15 years. Erythrocyte Na/K flux ratio was negatively correlated with serum K level, although it showed no significant correlation with either serum Na, urinary Na and K excretions or fractional excretions of filtered Na and K. These findings suggest that erythrocyte Na/K flux ratio, which has been reported to be abnormal under some pathological conditions including essential hypertension, may depend on serum K level, and that the flux ratio may not have a role in the renal Na or K control mechanism.