To evaluate the role of glomerular hyperfiltration in the development and progression of diabetic nephropathy, we performed clearance and histopathologic studies in 24 rats with streptozocin-induced diabetes after 3 months of diets with different protein compositions. Calcium phosphate was added to an 8% protein diet in group I (nine rats), and calcium carbonate to a 24% protein diet in group II (nine rats) to equalize calcium and phosphate contents in these diets. Group I and II rats also received small doses of insulin to reduce the excessive hyperglycemia induced by the high sucrose content of the diets. In group III, six rats given an 8% protein diet, no calcium, phosphate, or insulin was added. In groups I and III, low dietary protein significantly reduced glomerular filtration rate and renal plasma flow per gram of kidney weight as compared with rates observed in group II rats with a higher protein intake. Features of diabetic glomerulopathy including mesangial hypercellularity and mesangial matrix expansion were also significantly milder in the groups with a low protein diet. On the other hand, medullary calcification and interstitial changes were most prominent in group I, given calcium phosphate supplement; the increase in the kidney weight was greater in groups I and II, which received insulin, than in group III, which did not. It was concluded that low protein diet significantly ameliorates diabetic glomerulopathy but that supplementation with inorganic phosphate in an amount equal to organic phosphate contained in the higher protein diet causes medullary calcification and interstitial nephritis. Also, administration of suboptimal doses of insulin in diabetic animals greatly enhances renal growth, more than that induced by diabetes alone.