In 33 normotensive probands with a familial disposition to hypertension and in 18 normotensives without a family history of hypertension, aged 15 to 24 years, intracellular Na+ and Ca2+ activity in red blood cells was determined by ion-selective electrodes. The activities represent the free, i.e. unbound intracellular fraction of each ion. In the group with a family history of hypertension the mean intracellular Na+ activity was 9.74 +/- 1.43 mmol/l and clearly exceeded that in the group without predisposition to hypertension, which was 7.74 +/- 1.06 mmol/l. Even larger differences were found in mean intracellular Ca2+ activity (9.54 +/- 9.56 mumol/l in the group with familial disposition and 1.87 +/- 3.47 mumol/l in the control group). However, due to the relatively wide range of the values, especially with respect to the Ca2+ activities in probands with familial disposition to hypertension, the values from the two groups overlapped. The results suggest that elevation of intracellular Na+ and Ca2+ activity may be evidence for a genetically determined later development of hypertension.