Carefully age-matched, purebred male beagle dogs that underwent uninephrectomy one month after they were made diabetic with alloxan were used to establish a model of rapidly developing diabetic nephropathy in a large animal. The diabetic animals, all requiring insulin, were divided into two groups: one group with control by insulin injections permitting elevated fasting and postprandial serum glucose values and substantial glycosuria; the other with better control and with near-normal serum glucose levels and less glycosuria. By 1 year of diabetes both diabetic groups had renal lesions different from the uninephrectomized control animals but differing only slightly from one another. With light microscopy, diabetic dogs had increased mesangial thickening. With electron microscopic morphometry, glomeruli of diabetic subjects demonstrated increased fractional volumes of the total mesangium and of its cellular and matrix components and increased width of the GBM. These quantitative measures of diabetic nephropathy in the dog within 1 year of onset of the disease describe a model potentially useful in evaluating the efficacy of improved diabetic control in preventing or ameliorating diabetic nephropathy.