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To prove the effect of aging on the synthesis of renin-angiotensin-aldosterone system, renal kallikrein-kinin system, prostaglandin (PG), and thromboxane (TX) which regulate blood pressure, normotensive subjects and patients with essential hypertension (EH) were investigated in the present study. Although plasma renin activity (PRA) and plasma aldosterone concentration (PAC) were decreased with aging in both groups, there is no significant differences between each groups while compared among each age-groups. Urinary excretion of kallikreins (active, inactive and total) in EH were decreased with aging as similar extent to that of normal subjects. There was no age-related change of kallikrein activation ratio in EH in contrary to normal subjects. In comparison with each age-group, the amount of urinary kallikrein excretion in EH were already small in young age-groups. The amount of urinary PGE2 excretion in female EH group was smaller than that of normal subjects, and there were no age-related changes in both groups. Urinary excretion of TXB2 and 11-dehydro-TXB2, which are the urinary major metabolites of TXA2 which has potent vasoconstrictive action, were increased in the age-group of 80-93 year-old both normal subjects and EH. There were no age-related change in both groups. Although these hypertensive vasoactive substances as renin, aldosterone and TXA2 in EH show the same profile as that in normal subjects, the synthesis of renal antihypertensive vasoactive substances as kallikrein and PGE2 in EH already decrease in younger patients. These results suggest that the lower activities of renal antihypertensive system is a cause of the development of hypertension.