To examine the renal effects of lifelong intermittent feeding, we performed clearance and pathologic studies in 86 week old, awake male Sprague-Dawley rats fed on alternate days (N = 9) or ad libitum (N = 8) since the age of four weeks. Alternate day-fed rats were studied on both feeding and fasting days, and the values averaged. In the alternate day group the clearances of inulin (Cinulin) and PAH (CPAH), factored by body wt, were higher by 23% and 27%, respectively (P less than 0.05); albumin excretion (UalbV) was two orders of magnitude lower (P less than 0.001) and the percentage of glomeruli with lesions was eightfold lower (P less than 0.02) than in the ad libitum-fed group. The fractional clearances of neutral dextrans ranging in radii from 20 A to 42 A did not differ between the two groups. Compared to a previously published study of 30 week old, alternate day-fed rats, values for Cinulin and CPAH were similar while UalbV was higher (P less than 0.025) in the 86 week old alternate day-fed rats. Cinulin, however, was lower (P less than 0.005) while UalbV was much higher (P less than 0.001) in 86 week old, ad libitum-fed rats than in 30 week old, ad libitum-fed rats. The results indicate that long-term alternate day feeding preserves glomerular filtration rate (GFR) and renal plasma flow (RPF), while glomerular permselectivity is not completely preserved, as evidenced by an increase in microalbuminuria in aging awake male rats. Conversely, ad libitum feeding results in a significant decline in GFR and probably in RPF, in association with massive albuminuria and segmental glomerular sclerosis.