We have previously reported the production of monoclonal antibodies (MAb) recognizing the core protein of a basement membrane-specific chondroitin sulfate proteoglycan (BM-CSPG). Using immunohistochemical techniques, we have shown that BM-CSPG is present in almost every basement membrane, one exception being the normal glomerular capillary basement membrane (GBM), where it is absent. In the present study of mature kidneys we examined the distribution of BM-CSPG in streptozocin-induced diabetes mellitus in rats. We found BM-CSPG atypically associated with the GBM of diabetic animals as early as 1 month after induction of diabetes mellitus. Immunoelectron microscopy (IEM) of affected capillary loops showed BM-CSPG present in the subendothelial matrix in areas of GBM thickening and absent in areas where the GBM appears to be of normal thickness. Moreover, the association of BM-CSPG with regions of the pericapillary GBM affects the morphology of the capillary endothelial cells within these areas, directly displacing the cell body from the GBM proper and causing loss of fenestrae. These new data on BM-CSPG distribution reflect abnormal glomerular extracellular matrix protein biosynthesis/turnover in diabetes and suggest that BM-CSPG in the GBM might in turn affect normal capillary structure and/or function.