The present study was conducted to elucidate the mechanism (s) of the development of early diabetic nephropathy, examining ultrastructural changes employing electron microscopy, especially changes in pore size of the glomerular basement membrane (GBM) of streptozotocin (STZ)-induced diabetics rats. Urinary albumin excretion rate (UAE), pore size of the lamina densa of the GBM visualized directly by the tissue negative staining method, and number of anionic sites (AS) in the corresponding portion of the lamina rara externa were determined for 6 weeks in diabetic rats without and with insulin treatment. The UAE of the diabetic rats increased with time and was significantly greater than that of the nondiabetic control rats after 4 weeks (p< 0.01), while insulin treatment suppressed the increased UAE of diabetic rats. The median values in both short diameter and long dimension of the pores in the diabetic group were markedly increased at the 2nd week as compared with those in the nondiabetic control rats, whereas no significant change was found in the pore size of the diabetic rats with insulin treatment. Moreover, the number of AS in the GBM of the diabetic rats was significantly (p< 0.001) decreased from the 2nd week onward. Insulin treatment also prevented a decrease in AS number in diabetic rats. It is suggested from these results that an impairment of barrier size selectivity occurs at a very early stage of STZ-induced diabetes in rats, which may enhance the abnormality of the charge-selective properties of the GBM. In addition, insulin treatment may protect this barrier system through normalizing blood glucose control in STZ-diabetic rats.