Supine and upright plasma renin activity (PRA) were measured i 104 normal subjects (age range 13-74 years, 71 males and 33 females) on a constant diet of normal potassium (60-80 mEq/day) and of varying but constant, sodium content (range 10-260 mEq/day). Both supine and upright PRA values were inversely related to the 24-hour urinary sodium excretion, while only upright PRA values showed a significant inverse correlation with age. Through a multiple regression analysis it was shown that age increased the significance of the PRA/sodium relationship by about 10%. Besides posture, two other factors seem to influence the PRA-age relationship: the age range and the sodium intake. In fact, the PRA-age relationship was not detectable either when subjects below 20 and above 50 were excluded, or when sodium intake was increased above 140 mEq/day. These data can explain the contrasting reports on the age influence on renin secretion. The relationship between PRA and urinary sodium confirms the dependance of PRA on the state of sodium balance. Age significantly influences the PRA/sodium relationship of normal subjects on normal or low sodium intake and in the upright position. Therefore, the decline of PRA with age may be explained by the decrease either of renin storage or of sympathetic nervous system activity on renin release.