The relation between aging and plasmatic renin activity (PRA) and plasmatic aldosterone (PA) was studied in three groups of healthy normotensive male subjects: group A (subjects aged 20-40 years), group B (subjects aged 41-60 years), group C (subjects aged 61-80 years). Each group consisted of 15 subjects. All studied subjects were kept for a week on standardize life conditions with a defined daily intake of 120 mEq of sodium and 60 mEq of potassium. Venous blood samples were collected at 8.00 a.m., the subjects resting in supine position at least eight hours before the study. PRA and PA levels were determined by radioimmunoassay. The mean value of PRA and PA in the three groups were compared by t Student test. The results show that both PRA and PA present a decrease with age advancing: in fact the levels of PRA and PA are significantly higher in group A versus group B and group C, and in group B versus group C. The decline in PRA levels in the older subjects appeared to be due to various factors. It is concluded that aging process causes a decrease in PRA, with parallel lowering of PA and that these modifications have to be considered in biochemical diagnostic work-up of patients with arterial hypertension.