Glomerular filtration rate (GFR) was measured by constant infusion of iothalamate in conscious rats with and without streptozotocin diabetes. Kidney weight had already increased significantly after 4 days of diabetes. GFR was unchanged at that time, but increased subsequently, so that GFR/kidney weight was normal after 1 month of diabetes. Tubular glucose reabsorption rate was increased to the same extent as kidney weight after 4 days of diabetes. It is concluded that renal growth in experimental diabetes cannot be initiated by glomerular hyperfiltration, but that increased glucose reabsorption may have a causal role.