The effect of angiotensin I-converting enzyme inhibition (ACEI) on the age-related changes in the kidney structure and function was investigated in rodents. Normotensive male Wistar (WAG)/Rij rats were treated with perindopril from the age of 6 mo to the day of killing at 12, 24, or 30 mo. Mean blood pressure, constant from 6 to 30 mo, was reduced by 19 mmHg in treated animals. With age, the major functional modifications were a decrease in glomerular filtration rate and in renal blood flow, a rise in intrarenal vascular resistance (IVR), a reduced tubular reabsorption of salts, and a progressive increase in proteinuria. ACEI significantly reduced IVR and proteinuria. From a structural point of view, the glomeruli showed 1) an increase in size, 2) a decrease in capillary surface, 3) a diffuse thickening of the glomerular basement membrane, 4) an expansion of the mesangial matrix, and 5) an accumulation of albumin droplets in podocytes inducing 6) a dispersed focal and segmental glomerulosclerosis which, at 30 mo, affected < 2% of glomeruli. Of these six age-related structural changes, ACEI delayed the appearance of the three latter changes.