Exchangeable sodium is lower than normal in young male patients with essential hypertension. This may reflect a primary abnormality of sodium metabolism, or natriuresis caused by sodium-independent elevation of arterial pressure. To investigate this question, 31 normotensive men with positive and 31 normotensive men with negative family history of essential hypertension were studied. Blood pressure tended to be higher in the former (121/78 ± 9/8 (s.d.) versus 113/74 ± 11/9mmHg; P < 0.005); mean age, urinary sodium or potassium excretion, plasma sodium, potassium, renin activity or aldosterone levels and creatinine clearance were comparable. Exchangeable sodium and blood volume were also similar in the two groups, when expressed in absolute values (3113 ± 306 versus 3044 ± 242 mmol and 4902 ± 581 versus 4769 ± 579 ml, respectively) or in relation to body surface area (100.8 ± 7.1 versus 100.2 ± 6% and 103.8 ± 12.2 versus 102 ± 11.3%). In both groups, exchangeable sodium and blood volume were unrelated to arterial pressure. The body sodium/blood volume state is normal in normotensive subjects with positive family history. The low exchangeable sodium of young patients with essential hypertension does not appear to reflect a primary familial abnormality of body sodium metabolism.