Activation of endothelin (ET) signaling in vascular wall plays an important role in the development of secondary atherosclerosis, including re-stenosis post-angioplasty and transplantation vasculopathy. We have shown previously that ET activates vascular smooth muscle cell (VSMC) proliferation through increased expression of the key cell cycle phosphatase, cell division cycle 25 homolog A ( Cdc25A). In this study, we asked whether a selective ET A receptor antagonist, atrasentan is able to block injury-induced in vivo VSMC proliferation associated with reduction of Cdc25A expression. After measurement of blood pressure (BP) and 24-hour urine sodium excretion, 18 male Sprague-Dawley rats were randomly divided into 3 groups to receive vehicle or atrasentan (5 mg/kg/d) in the drinking water 7 days prior to sham surgery ( control group) or the right common carotid artery (CCA) ligation ( with or without atrasentan groups). The treatment was continued for 21 days after operation. At that time their BP and urine sodium excretion were measured again before sacrifice. There was no significant difference in BP and 24-hour urine sodium excretion among 3 groups before and 21 days after surgery. CCA ligation led to neointimal hyperplasia. Treatment with atrasentan resulted in about a 75% reduction of the hyperplasia. The neointima dominantly consisted of VSMCs confirmed by anti-smooth muscle (SM)-α-actin staining. To determine whether CCA ligation induces Cdc25A expression in VSMCs and this induction is blocked by atrasentan, we conducted co- immunofluorescence assay with anti-SM-α-actin and anti-Cdc25A antibodies. The ligation markedly increased Cdc25A expression in VSMCs and the expression was much higher in neointimal VSMCs than those in medial layer. Atrasentan reduced ligation-mediated Cdc25A expression in neointimal VAMCs by ~80%, while it seemed not significantly decrease VSMC Cdc25A levels in medial layer. These results demonstrate that atrasentan inhibits VSMC proliferation in vivo possibly through a Cdc25A-dependent mechanism and suggest that this compound may prove useful in the management of disorders characterized by acute aberrant proliferation of VSMCs in the vascular wall.