Although Smad3 is a key mediator of fibrosis, the functional role of Smad3 in hypertensive cardiovascular disease remains unclear. The present study tested the hypothesis that angiotensin II may activate the transforming growth factor-beta/Smad3 pathway to mediate hypertensive cardiac remodeling in Smad3 knockout (KO) and wild-type mice by subcutaneous angiotensin II infusion and in the primary culture of Smad3 KO cardiac fibroblasts. Fourteen days after angiotensin II infusion, both Smad3 KO and wild-type mice developed equal levels of high blood pressure. However, hypertensive cardiac fibrosis and inflammation were developed in Smad3 wild-type but not in Smad3 KO mice. This was demonstrated by the findings that mice lacking Smad3 were protected against a fall in left ventricular ejection fraction (P<0.05), an increase in left ventricular mass (P<0.05), and the development of cardiac fibrosis and inflammation, including upregulation of transforming growth factor-beta1, connective tissue growth factor, collagen I/III, alpha-smooth muscle actin, interleukin 1beta, tumor necrosis factor-alpha, monocyte chemoattractant protein 1, intercellular adhesion molecule 1, and an increase in macrophage and T-cell infiltration in left ventricular tissues (all P<0.01, respectively). Additional studies in vitro also revealed that angiotensin II-induced cardiac fibrosis and inflammation were prevented in Smad3 KO cardiac fibroblasts. Inactivation of both Smad3 and nuclear factor kappaB/p65 signaling pathways was a key mechanism by which Smad3 KO mice were protected from angiotensin II-mediated hypertensive cardiac remodeling. In conclusion, Smad3 plays an essential role in hypertensive cardiac remodeling. Results from this study suggest that targeting Smad3 may be a novel therapeutic strategy for hypertensive cardiovascular disease.