1. During 4 weeks 37 normotensive 50-year-old men identified by screening in a random population sample were given 12 g of NaCl daily, in addition to their usual dietary sodium intake. Blood pressure, heart rate, weight, urinary excretion of sodium, potassium and catecholamines, plasma aldosterone and noradrenaline and intra-erythrocyte sodium content were determined on normal and increased salt intake. The subjects were divided into those with a positive family history of hypertension ( n = 11) and those without such a history ( n = 26). 2. Systolic blood pressure and weight increased significantly irrespective of a positive family history of hypertension. 3. On normal salt intake intra-erythrocyte sodium content was significantly higher in those with a positive family history of hypertension. During high salt intake intra-erythrocyte sodium content decreased significantly in that group and the difference between the hereditary subgroups was no longer significant. 4. In the whole group urinary excretion of noradrenaline, adrenaline and dopamine increased whereas plasma aldosterone decreased during the increased salt intake. 5. Thus, in contrast to some earlier studies performed in young subjects, our results indicate that moderately increased sodium intake acts as a pressor agent in normotensive middle-aged men whether there was a positive family history of hypertension or not. We confirm that men with positive family history of hypertension have an increased intra-erythrocyte sodium content, and that an increase in salt intake seems to increase overall sympathetic activity.