Mammalian cardiocytes secrete atrial natriuretic peptides (ANPs) into plasma, which cause marked natriuresis, diuresis, vasorelaxation and inhibition of hormone secretions. Aging influences the ability of the kidney both to conserve and to excrete sodium; i.e., in old animals, the excretory capacity of sodium is reduced and the time required to excrete sodium load is prolonged. Therefore, it is possible that animals differing in ages may respond differently to ANP. In the present study, we determined whether the renal, hormonal and vascular effects of ANP may be influenced by aging in conscious rabbits. The plasma renin concentration decreased with aging but plasma ANP concentration was significantly lower only in 24-month-old rabbits. Plasma aldosterone concentration and atrial ANP content did not change by aging. In 1-month-old rabbits, ANP (atriopeptin III, 3 ug/kg) administered intravenously caused hypotension and decreased in plasma renin and aldosterone concentrations, but did not cause diuresis and natriuresis. In 2 to 5 month-old rabbits, ANP caused hypotension, decreases in Plasma renin and aldosterone concentrations and marked renal effects. However, in 24-month-old rabbits, all the above effects of ANP was blunted. With hydration of physiological saline at a rate of 15 ml/kg/h for 2hr, urine volume and glomerular filtration rate did not change but the electrolyte excretion as well as fractional excretion of sodium significantly increased. The plasma concentrations of active renin and aldosterone were decreased but plasma inactive renin and ANP concentrations were increased. The changes in renal function and plasma level of hormone showed no differences in different ages. These results suggest that the peripheral vascular receptors to ANP may develop earlier than those in the kidney, and the attenuated vascular and renal responses to ANP in the old age may be due to age-related modifications in renal function and blood vessel.