Background: Endoglin is a membrane glycoprotein that regulates TGF-β1 signaling. Previous studies have revealed that endoglin is upregulated in several models of experimental fibrosis, and that endoglin expression can counteract the fibrogenic effects of TGF-β1. As treatment with angiotensin converting enzyme (ACE) inhibitors reduces renal fibrosis by mechanisms that are, in part, not dependent on angiotensin II blockade, we have assessed the hypothesis that this effect could be mediated by endoglin upregulation. Methods: We have used the 5/6-nephrectomy renal mass reduction (RMR) model of renal fibrosis in rats treated (RMR+T) or not treated with the ACE inhibitor trandolapril (0.7 mg/kg/day). One, 3 and 5 months after RMR, mean arterial pressure and renal function were measured. In addition, renal fibrosis was evaluated quantitatively and endoglin, TGF-β1, collagen type I and collagen type IV expression was assessed by Northern blot and immunohistochemistry. Results: RMR induced a progressive increase in mean arterial pressure, urinary protein excretion and glomerular and tubulointerstitial fibrosis, which is accompanied by an increased expression of TGF-β1, endoglin and collagen types I and IV. Trandolapril treatment reduced systemic blood pressure and lessened proteinuria after RMR, as well as expression of TGF-β1, endoglin and collagens. Conclusion: The present study demonstrates an increased TGF-β1, endoglin, collagen type I and collagen type IV expression in rats with severe hypertension and renal damage. The effect of trandolapril to decrease renal fibrosis seems to be based in a reduced TGF-β1 expression but not in an increased expression of endoglin.