Compared with white normal males without a family history of hypertension (n=43), red blood cells of white normal males with such a family history (n=17) had a higher (P<0.01) intracellular sodium concentration (8.07 ± 0.30 versus 7.33 ± 0.17 mmol/l cells). This could be at least partly due to their lower (P<0.001) frusemide-sensitive sodium efflux rate (250 ± 19 versus 424 ± 23 µmol/l cells/h). Also their ouabain-resistant 86Rb-uptake (0.81 ± 0.06 versus 0.97 ± 0.03 µmol/l cells/h) was lower (P<0.05). Potassium concentration in the red blood cells was similar in both groups.