SummaryCompared to white normal males without family history of hypertension (n = 43) red blood cells of white, normal males with such a history (n = 17) have a higher (p<0.01) intracellular sodium concentration (8.07±0.30 mmol/1 cells versus 7.33±0.17 mmol/1 cells). This could be at least partly due to their lower (p<0.001) furosemide-sensitive sodium efflux rate (250±19 /umol/l cells/h versus 424±23,1/mol/l cells/h). Also their ouabainresistant86Rb-uptake (0.81±0.06 /imol/1 cells/h versus 0.97±0.03 /imol/1 cells/h) was lower (p<0.05). The potassium concentration in the red blood cells was similar in both groups.Compared to white males without family history of hypertension (n = 43) red cells of black males without such a history (n = 12) have also a higher (p<0.001) intracellular sodium concentration (13.42±1.7 mmol/1 cells). This could result from their lower (p<0.001) furosemide sensitive sodium efflux and their lower Na K+-ATPase activity, as estimated from the ouabain-sensitive86Rbuptake.Our results indicate that the intra-erythrocyte Na+concentration and transmembrane Na, K+-cotransport activity and Na+, K+-ATPase activity are different according to race and family background of hypertension.