The present study was carried out to examine the effect of .L-arginine on advanced stage nonenzymatic glycosylation end products in glomerular basement membrane (GBM) as represented by carboxymethyllysine (CML). Twelve db/db mice were given a solution containing a daily dosage of L-arginine of 50 mg/kg body weight orally. Twelve db/db mice served as controls. At the end of the 4-months study period treated animals had significantly lower concentrations of CML (0.084 ± 0.008, 0.071-0.098 nmol/µmol OH-proline; mean ± SD, range, p < 0.01) compared to untreated controls (0.11 ± 0.018, 0.095-0.152 nmol/ µmol OH-proline). In addition, there was a significant positive correlation between GBM thickness and concentrations of CML (r = 0.86, p < 0.001). We conclude that reduction of CML concentrations in treated db/db mice possibly reflects a beneficial effect of L-arginine on advanced stage nonenzymatic glycosylation endproducts in GBM. In addition, measuring CML concentrations might have future clinical implications as a noninvasive parameter for basement membrane thicke