In rats with chronic diabetes, the synthesis of glomerular basement membrane was investigated in vivo by injection of 3H-proline. Compared to the nondiabetic controls, the basement membrane synthesis was markedly raised. The formation of hydroxyproline as parameter for the synthesis of the collagen portion of the basement membrane increased to a somewhat greater extent than the incorporation of 3H-proline. Hydroxyproline formation and proline incorporation showed a close correlation with the blood sugar level in the diabetic rats.