Diabetic nephropathy is a feared complication of diabetes since it can lead to end-stage renal failure and also it is a risk factor of cardiovascular disease. The important clinical problems caused by diabetic nephropathy are proteinuria and decreased renal function. Exercise is a cornerstone of diabetes management, along with diet and medication. Since acute exercise causes proteinuria and decreases glomerular filtration rate, the effect of exercise on diabetic nephropathy is controversial. The aim of this study was to investigate the effect of regular aerobic exercise on microalbuminuria and glomerular filtration rate in diabetic rats. Moderate diabetes was induced by streptozotocin (45 mg/kg IV) in rats and an aerobic exercise-training program on a treadmill was carried out for 8 weeks. Four groups of rats; control sedentary (CS), control exercise (CE), diabetic sedentary (DS) and diabetic exercise (DE) were included in the study. Blood glucose levels were determined from the plasma samples taken at the end of 4 weeks of stabilization period and 8 weeks of training program. Creatinine clearance (C-Cr) and microalbuminuria (MA) levels were determined to evaluate renal functions. The analyzed data revealed that regular aerobic exercise: 1) significantly decreased the plasma glucose level of the DE group compared to the DS group (p < 0.05), 2) significantly decreased the microalbuminuria level of the DE group compared to those of DS group (p < 0.01), 3) significantly decreased the creatinine clearance levels of the DE and CE groups compared to those of CS group (p < 0.05). The results of this study suggest that despite of decreasing creatinine clearance, regular sub-maximal aerobic exercise has a preventive effect on development of microalbuminuria and thus may retard nephropathy in diabetic rats.