Background/Aims: Glomerular basement membrane (GBM) thickening and mesangial matrix expansion are characteristic features of diabetic nephropathy. The present study investigates the role of endothelins (ETs) in the pathogenesis of such changes in diabetic nephropathy. Methods: Diabetic (streptozotocin-induced, 65 mg/kg), galactose-fed (30%) and control animals were followed up for 1 and 6 months. Animal groups also included diabetic and galactose fed animals on dual ET<sub>A</sub>/ET<sub>B</sub> receptor antagonist bosentan (100 mg/kg). A semi-quantitative reverse transcription polymerase chain reaction method was used to quantify mRNA expression of ET-1, ET-3, ET<sub>A</sub>, ET<sub>B</sub>, fibronectin and collagen α2(IV). Histological analyses of the kidneys and ET-1, ET-3 and fibronectin immunohistochemistry were performed. Morphometric assessment of the GBM after 6 months was performed. Results: Diabetes increased mRNA expression of ET-1, ET-3, ET<sub>A</sub>, ET<sub>B</sub>, fibronectin and collagen α2(IV) after one and six months. In contrast, although increased ET<sub>A</sub> and ET<sub>B</sub> mRNAs were present following galactose feeding both at 1 and 6 months, ET-1, ET-3, fibronectin and collagen α2(IV)mRNAs were increased after 6 months. Both diabetes and galactose feeding caused increased GBM thickening. Furthermore, diabetes caused an increase in mesangial matrix production. Bosentan prevented increased fibronectin and collagen α2(IV) mRNA expression, increased mesangial matrix deposition and GBM thickening. Conclusion: This study has demonstrated that diabetes and galactose feeding induced functional and structural changes in the kidney are mediated via ETs.