Abstract: AIM To explore the role of angiotensin II (AngII) in the collagen synthesis of cardiac fibroblasts and the putative involvement of miR-21. METHODSPrimary cardiac fibroblast cells were isolated from Sprague Dawley rats and the cells were treated with 0, 0.1, 0.2 and 0.4 μmol/L AngII for 36 h. Expressions of Col1a1, Col3a1 and miR21 were analyzed by qRT-PCR. MiR-21 was knocked down by transfected miR-21 inhibitor into cardiac fibroblast cells. RESULTSCardiac fibroblast cells were successfully isolated and cultured as seen morphologically under the microscope and expression of the markers. Upon AngII treatment, expressions of Col1a1, Col3a1 and miR21 increased in a dose-dependent manner. miR21 inhibitor markedly blocked the induction of Col1a1 and Col3a1 by AngII. CONCLUSIONAngII induces collagen synthesis in which induction of miR-21 might be essential.