This study evaluated the relationship between the development of fluorescence related to advanced glycosylation end products (AGEs) in the kidney and experimental diabetic nephropathy over a 32-wk period. Control, untreated diabetic, and aminoguanidine-treated diabetic rats were followed for 32 wk with eight weekly measurements of urinary albumin excretion. After 32 wk, collagen-related fluorescence in aorta and kidney (whole kidney, isolated glomeruli, and renal tubules) and glomerular ultrastructure were evaluated. Diabetes was associated with a significant increase in collagen-related fluorescence in the aorta and kidney. Aminoguanidine prevented the increases in collagen-related fluorescence in aorta, isolated glomeruli, and renal tubules but not in whole kidney. Diabetes was associated with increased albuminuria, fractional mesangial volume, and glomerular basement membrane (GBM) thickness. Aminoguanidine attenuated the rise in albuminuria and prevented mesangial expansion without influencing GBM thickness in diabetic rats. The concomitant changes in collagen-related fluorescence, albuminuria, and mesangial expansion with aminoguanidine therapy are consistent with the hypothesis that AGEs may play a role in the development of diabetic nephropathy.