The effects of experimental diabetes on renal tubular Na,K-ATPase activity were measured 4, 7, 14, 21, 28, and 56 days after ip injection of streptozotocin (STZ; 60 mg/kg) in rats. Significant increases in serum and urinary glucose levels as well as urinary volume and electrolyte output were observed 24 h after STZ injection. The elevated serum and urinary glucose levels were maintained during the entire 8-week experimental period, while urinary volume and electrolyte output showed an initial rising phase, reaching a peak at approximately 2–3 weeks, followed by a stabilization phase at a level lower than the peak effect, but still significantly higher than that in the salinecitrate-treated controls. A significant increase (+25.3%) in renal outer medullary Na,K-ATPase activity was observed 4 days after the induction of STZ-diabetes, while similar increases were not observed in the cortical regions until after 7 days of experimental diabetes. These elevated renal cortical and outer medullary enzyme activities, however, were subsequently maintained during the entire 8-week experimental period. In addition, a similar time course of development of renal hypertrophy, as indicated by increases in kidney weights and kidney protein to DNA ratios, was observed after the induction of STZ-diabetes in rats. Therefore, the present data indicate that renal hypertrophy and increased renal Na,K-ATPase develop early and with a similar time course after induction of STZ diabetes in rats and may mediate the gradual amelioration of excessive renal electrolyte loss seen in this experimental condition. Since Na,K-ATPasemediated ion transport is the major consumer of metabolic energy in the kidney and is centrally important to renal function, it is suggested that the early and pronounced increase in renal Na,K-ATPase seen in diabetes is an essential component of the renal hypertrophy and hyperfunction seen in this disease and may represent an important adaptive change in the kidneys in response to glucose osmotic diuresis in the experimental diabetic animals. © 1986 by The Endocrine Society.