The progression of anatomic and functional changes that occur with aging in normal male Sprague-Dawley rats was investigated. Renal function studies were followed by vascular perfusion fixation of the kidneys. The kidneys were examined with light microscopy and scanning and transmission electron microscopy. The glomerular filtration rate of 12-month-old rats (0.30 +/- 0.06 ml/min/100 gm body weight) was significantly lower (P less than .05) than the rates of the 3-month-old (0.91 +/- 0.08 ml/min/100 gm body weight) and 5-month-old (0.99 +/- 0.16 ml/min/100 gm body weight) rats. Urine protein levels were moderately elevated in 12-month-old rats. Structural changes mainly involved the renal corpuscle and proximal tubule. The parietal layer of Bowman's capsule was transformed from a squamous to a columnar epithelium resembling that of the proximal tubule in 48% of the capsules surveyed in 12-month-old rats. This value was significantly higher (P less than .05) than that of the 5- (14%) or 3- (6%) month-old rats. Thin sheets of cytoplasm extended from podocytic cell bodies over the layer of pedicels. The proximal tubule displayed focal areas of cell injury and necrosis. Basement membranes associated with the renal corpuscle, glomerulus, and proximal tubule were considerably thickened in the oldest group of rats. Some cellular infiltration occurred in the interstitium. Therefore, kidneys of aging Sprague-Dawley rats exhibited capsular metaplasia, death, and injury of proximal tubular epithelial cells, and other structural changes not seen in younger animals. Some of these changes may underlie the functional deterioration of their kidneys.