Background Cardiac fibrosis is a core pathological process associated with heart failure. The recruitment and differentiation of primitive fibroblast precursor cells of bone marrow origin play a critical role in pathological interstitial cardiac fibrosis. The K3.1 channels are expressed in both ventricular fibroblasts and circulating mononuclear cells in rats and are upregulated by angiotensin II . We hypothesized that K3.1 channels mediate the inflammatory microenvironment in the heart, promoting the infiltrated bone marrow-derived circulating mononuclear cells to differentiate into myofibroblasts, leading to myocardial fibrosis. Methods and Results We established a cardiac fibrosis model in rats by infusing angiotensin II to evaluate the impact of the specific K3.1 channel blocker TRAM -34 on cardiac fibrosis. At the same time, mouse CD 4 T cells and rat circulating mononuclear cells were separated to investigate the underlying mechanism of the TRAM -34 anti-cardiac fibrosis effect. TRAM -34 significantly attenuated cardiac fibrosis and the inflammatory reaction and reduced the number of fibroblast precursor cells and myofibroblasts. Inhibition of K3.1 channels suppressed angiotensin II -stimulated expression and secretion of interleukin-4 and interleukin-13 in CD 4 T cells and interleukin-4- or interleukin-13-induced differentiation of monocytes into fibrocytes. Conclusions K3.1 channels facilitate myocardial inflammation and the differentiation of bone marrow-derived monocytes into myofibroblasts in cardiac fibrosis caused by angiotensin II infusion.C