Rho-kinase is an effector molecule of RhoA, a monomeric GTP-binding protein, and causes Ca(2+) sensitization via inactivation of myosin phosphatase. The major physiological functions of Rho-kinase include contraction, migration, and proliferation in cells. These actions are thought to be related to the pathophysiological features of asthma, i.e., airflow limitation, airway hyperresponsiveness, beta-adrenergic desensitization, eosinophil recruitment and airway remodeling. Here, the roles of RhoA/Rho-kinase in the pathophysiology and treatment of asthma were investigated. In airway smooth muscle, pre-exposure to chemical mediators released from inflammatory cells markedly enhances methacholine-induced contraction without elevating intracellular concentrations of Ca(2+). This augmented responsiveness to methacholine involves the phosphorylation of myosin phosphatase targeting protein 1 (MYPT1) via Rho-kinase, however, it is attenuated by pre-treatment with Rho-kinase inhibitors such as Y-27632 and HA-1077. Airway smooth muscle contraction due to asthma-related substances such as contractile agonists and reactive oxygen species is suppressed by these Rho-kinase inhibitors. Reduced responsiveness to beta-adrenergic receptor agonists occurs via Ca(2+) sensitization, after exposure to lysophospholipids and proteases released from inflammatory cells. This beta-adrenergic desensitization is also attenuated in the presence of Y-27632. Furthermore, the proliferation of airway smooth muscle cells is elevated by Rho-kinase, however, it is markedly suppressed by Y-27632. Antigen challenges cause hyperresponsiveness and eosinophilia in the airways; however, these reactions are markedly suppressed by these Rho-kinase inhibitors. These findings indicate that RhoA/Rho-kinase is involved in the pathophysiology of asthma, and suggest that Rho-kinase inhibitors have therapeutic potential for prohibiting these features. In conclusion, RhoA/Rho-kinase is a novel target molecule for the treatment of asthma.