Danshensu, the effective ingredient of the plant Salvia miltiorrhiza (Danshen), has been widely used for treatment of cardiovascular diseases. Cardiac fibrosis is an important process in pathological cardiac remodeling and leads to heart failure. We investigated the effect of Danshensu on β-adrenergic receptor (β-AR)-mediated cardiac fibrosis and the involved signaling transduction. Danshensu inhibited cardiofibroblast proliferation and collagen I synthesis induced by isoproterenol (ISO), a selective β-AR agonist. Phosphorylation of p38 mitogen-activated protein kinase (MAPK), which mediates ISO-induced cardiac fibrosis, was negatively regulated in this process. The negative regulation depended on the ISO inhibition of reactive oxygen species (ROS) production. Taken together, Danshensu may inhibit β-AR-mediated cardiac fibrosis by negative regulation of ROS-p38 MAPK signaling.Key words Danshensu; β-adrenergic receptor; cardiac fibrosis; p38 MAPK; reactive oxygen specy Cardiac fibrosis is a pivotal phenomenon in pathological cardiac remodeling and is a hallmark of heart diseases. It is characterized by excessive extracellular matrix accumulation and fibroblast deposition and eventually destroys the organ architecture and abolishes normal function.1-3) Myocardial fibrosis is a major biological determinant of fatal events in cardiac remodeling, including heart failure, severe arrhythmias and sudden cardiac death.4,5) Cardiac fibrosis can be caused by various factors, such as the chronic activation of the sympathetic nervous system, myocardial hypoxia, ischemia, senescence, inflammation and hormones. β-Adrenergic receptors (β-ARs) and their associated guanine nucleotide regulatory protein (G protein)/adenylyl cyclase signal transduction pathways, are central to the regulation of cardiac function. Continuous activation of β-ARs plays an important role in cardiac dysfunction.6-8) Cardiac function was altered in β 1 -AR transgenic mice with cardiac hypertrophy, which could be inhibited by β-AR blockers.8) Consecutive administration with isoproterenol (ISO), a β-AR agonist, caused cardiac fibrosis in rats. 9,10) p38 mitogen-activated protein kinase (p38 MAPK) and reactive oxygen species (ROS) play a critical role in cardiac * To whom correspondence should be addressed. e-mail: lizijian@bjmu.edu.cn;zhengxh@nwu.edu.cnThe authors declare no conflict of interest. # These authors contributed equally to this work.