The age-related changes in the renin-angiotensin system were investigated in normotensive 3-, 10-, 20-, and 30-mo-old female Wistar rats. Plasma renin concentration and immunofluorescent renal renin index remained constant from 3 to 10 mo, then decreased as the animals become older, whereas plasma concentration of renin substrate diminished slightly between 10 and 20 mo and plasma converting-enzyme activity was not modified with age. Acute inhibition of converting-enzyme activity by intravenous administration of 0.1 mg/100 g body wt S 9780 (the diacid form of S 9490) was followed by a 7- to 8-mmHg decrease in arterial pressure and a concomitant 10-12% increase in inulin and p-aminohippuric acid (PAH) clearance in the 10- and 20-mo-old rats. On the other hand, neither glomerular filtration rate, PAH clearance nor arterial blood pressure were affected by converting-enzyme inhibition in the 30 mo-old rats. These results indicate that the activity of the renin-angiotensin system is progressively reduced with age and suggest that angiotensin II does not play an important role in the maintenance of blood pressure and kidney hemodynamics in normotensive female senescent rats.