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In streptozotocin induced diabetes in rats, excretion of urinary protein fractions were studied in relation to structural changes in the renal glomeruli, using light and transmission electron microscopy. After six weeks of induced diabetes only beta 1 and beta 2 plasma globulins were significantly elevated. The amount of excreted proteins and degree of glomerular changes were not proportional. In the initial stages (1-2 weeks) glomerular structural changes were very mild and were accompanied by significantly elevated proteinuria. This progressed (4-8 weeks) to moderate to prominent structural changes with intermittent proteinuria except for the fractions beta 1 & beta 2 which were elevated throughout the duration of the experiment. The amount of proteinuria was not proportional to changes in the plasma protein levels. The following conclusions may be made: 1) The mild early glomerular abnormalities seem to be mainly due to acute metabolic disturbances. 2) An early indication of diabetic nephropathy is provided not only by albuminuria, but may also be an elevated excretion of beta-globulin fractions. 3) Decrease of albuminuria in the later stages of diabetes may be related to the deposition of albumin as a basement membrane-like material in the mesangium.