In latent hereditary diabetes mellitus increased protein excretion has been found in male diabetic mice compared to controls. This proteinuria is partly due to an increased excretion of higher molecular weight proteins, which could be identified as deriving from plasma. The suggested glomerular proteinuria has been verified by an increase in the renal excretion of high molecular weight PVP. No changes in GFR occurred in this early stage of diabetic glomerulopathy. No changes in morphology could be detected, suggesting some changes in the biochemical membrane structure that cause the findings of increased permeability of the diabetic basement membrane for plasma proteins.