Micropuncture studies were made on insulin-treated streptozotocin diabetic rats two weeks after the induction of diabetes and on age-matched control rats. Kidney size, GFR and single nephron GFR were higher in poorly controlled diabetic rats than in normal animals. Single nephron GFR rose as a result of an increase in the hydraulic pressure difference across the glomerular capillary wall caused mainly by a rise in the glomerular capillary pressure due to a diminished ratio of afferent to efferent arteriolar hydraulic resistances. Furthermore, the intratubular pressure was reduced as a result of a decrease in hydraulic resistance in the loop of Henle. Strict metabolic control prevented these changes. In conclusion, the increase in renal function in experimental diabetes is determined by the degree of metabolic control excluding a potential nephrotoxic effect of streptozotocin per se.