Glomerular hemodynamics were measured in male Sprague-Dawley rats, aged 4 to 5 months (young) or 20 to 22 months (old). Body weight (BW) and left kidney weights (KW) were higher in old rats than young (BW: 507 +/- 12 g v 342 +/- 11 g, P less than 0.001; KW: 2.0 +/- 0.1 g v 1.3 +/- 0.1 g, P less than 0.001). Arterial blood pressure (AP) was slightly higher in old rats, but within the normotensive range (106 +/- 4 mm Hg v 94 +/- 4 mm Hg, P less than 0.05). Glomerular filtration rate (GFR; factored for KW) was lower in old versus young rats (0.67 +/- 0.05 mL/min/gKW v 1.00 +/- 0.08 mL/min/gKW, P less than 0.02). The cortical surface of the kidney in old (but not young) rats showed marked heterogeneity and single-nephron (SN)GFR was measured only in filtering nephrons and was higher and more variable in old versus young rats. Glomerular blood pressure (PGC) was unchanged in old compared with young rats (53 +/- 4 mm Hg v 55 +/- 2 mm Hg). There was a significantly greater level of glomerular sclerosis (in outer cortical glomeruli) in old versus young rats, and glomerular volume was substantially greater in old rats. This study suggests that age-related glomerulopathy is not primarily mediated by glomerular capillary hypertension.