Net sodium (Na) efflux and potassium (K) influx were determined in Na-loaded/K-depleted erythrocytes derived from 37 patients with essential hypertension and 25 age-matched normotensive subjects with no family history of hypertension, together with the measurement of basal red cell sodium and potassium contents. Intraerythrocyte sodium content was significantly higher in the essential hypertensives than in the normotensives (10.9 +/- 1.4 vs 10.0 +/- 1.2 mmol/L X cells, mean +/- SD, p less than 0.02), but potassium content was nearly equal between the two groups. Net Na efflux in the hypertensives was significantly reduced compared with that in the normotensives (4.57 +/- 0.70 vs 5.18 +/- 1.02 mmol/L X cells X hr, p less than 0.01), but both net K influx and net Na/K flux ratio were not significantly different between the two groups. Net Na efflux and K influx showed a significant inverse correlation with red cell sodium content (r = -0.64 and r = -0.56, respectively, p less than 0.001). These results suggest that the reduced net Na efflux with the increase of red cell sodium content may be related to the pathogenesis of essential hypertension. However, it is impossible to determine the genetic marker of essential hypertension by using the net Na/K flux ratio of Japanese subjects, although Garay et al. have reported that this index was abnormally low in the case of Europeans.