Morphologic characteristics of the renal glomeruli and tubules of BB rats with spontaneous diabetes mellitus were studied at 30 weeks' duration of diabetes. Whereas the glomerular basement membrane (GBM) was significantly thickened, no changes in the diabetic glomeruli were seen in the peripheral capillary wall area and in the fractional volumes of the mesangial cells or of the mesangial matrix. Light microscopy of the diabetic kidneys were normal, and immunofluorescent examination of diabetic glomeruli showed no increased accumulation of albumin, C3, or IgG. Diabetic rats had increased renal blood flow and glomerular filtration rates. Diabetic rats at 7, 17, and 30 weeks excreted normal amounts of urinary albumin. Thus kidneys of the BB diabetic rat differ from other experimental models of diabetes in that GBM thickening occurs in the absence of mesangial changes and of increased albuminuria. These studies suggest that the mesangium may influence glomerular permeability in diabetes, while thickening of the GBM in diabetes does not necessarily coincide with increased urinary albumin excretion. Furthermore, these results are consonant with the hypothesis that genetic factors may influence the pathological expression of diabetic nephropathy in rats.