Renal hypertrophy is a characteristic and early manifestation of diabetes in human and experimental animals. We examined the precise distribution of insulin-like growth factor-I (IGF-I) receptor mRNA in the experimental diabetic rat kidney using a nonradioactive in situ hybridization technique. Expression of IGF-I receptor mRNA was rarely seen in the glomeruli of control rats. IGF-I receptor mRNA was detected after induction of diabetes in glomerular mesangial, visceral epithelial, and parietal epithelial cells. The number of IGF-I receptor mRNA-positive cells in a glomerulus increased significantly at 4 weeks as compared with the control rats. Overexpression of IGF-I receptor in glomerular cells may contribute to the glomerular hypertrophy in diabetic nephropathy.