The diabetic-prone BioBreeding Wistar rat (BB/DP) is an autoimmune model of insulin-dependent diabetes mellitus. Approximately 80–90% of the animals are hyperglycemic (BB/DPh) by 90–120 days of age while those that do not become diabetic in adolescence (BB/DPn) remain normoglycemic for life. Likewise, rats in the diabetes-resistant (BB/DR) strain are normoglycemic. Although renal morphological studies have been carried out in this model, ultrastructural observations of age- and diabetes-related extracellular matrix (ECM) changes, including glomerular basement membrane (GBM) morphometry, are not available. Moreover, possible renal changes in the relatively uncommon BB/DPn control animals have not been reported. The current electron microscopic study was carried out to investigate temporal changes in detergent-treated acellular ECM in BB/DPh rats at 2 weeks, 3 months, 6 months, and 1 year postonset of moderate hyperglycemia. Age-matched BB/DR and BB/DPn control animals were also examined. Our data demonstrate age- and diabetes-related alterations in mesangial matrix distributions and GBM widths and show for the first time significant increases in GBM thickening in both hyperglycemic (BB/DPh) and normoglycemic (BB/DPn) rats when compared to age-matched BB/DR controls. Surprisingly, the rate of increase is greatest in BB/DPn animals. Although the pathogenesis of diabetic basement membrane disease is not completely understood, GBM thickening is widely regarded as a morphological consequence of hyperglycemia. However, data in the current investigation show that ECM alterations, including significantly increased GBM thickness, may occur in genetically diabetic animals in the absence of hyperglycemia. © 2004 Wiley-Liss, Inc.