Increased sympathetic nervous activity may play an important role in the pathogenesis of essential hypertension. It is well-known that both dietary sodium intake and age influence plasma norepinephrine (P-NE). There are many reports on comparative studies of P-NE in patients with essential hypertension and normotensive controls. However, there are few studies on changes in plasma catecholamines in both the aging and salt intake states. The present study was undertaken to evaluate the effect of age on sympathetic nervous activity in patients with essential hypertension and normal controls under low (UNa V 29 +/- 3 mEq/day), regular (UNa V 133 +/- 8 mEq/day) and high (UNa V 324 +/- 12 mEq/day) sodium regimens. The catecholamine levels were analyzed by THI methods after HPLC separation. Twenty-four hour urinary norepinephrine, epinephrine and electrolyte (Na+, K+) excretion on day 5 of each regimen was also determined. On day 6 morning, supine and upright P-NE, plasma epinephrine and plasma renin activity were determined after blood pressure and pulse rate measurement. Subjects were separated according to age as follows: young (less than 40 y.o.), middle-aged (40 approximately 60 y.o.) and old (greater than 60 y.o.). P-NE in hypertensives was significantly higher than in normals under any three sodium intakes. There was a significantly positive correlation between age and P-NE, but only in normal controls under three sodium intakes and not in hypertensive patients under any three sodium regimens. In the young group of hypertensive patients, P-NE was significantly higher than that of normal controls. These results suggest that the increased sympathetic nervous activity plays an important role in the pathogenesis and maintenance of essential hypertension, and this is particularly obvious in young patients.