During the development of hypertrophy, cardiac myocytes increase organization of the sarcomere, a highly ordered contractile unit in striated muscle cells. Several hypertrophic agonists, such as angiotensin II, phenylephrine, and endothelin-1, have been shown to promote the sarcomere organization. However, the signaling pathway, which links extracellular stimuli to sarcomere organization, has not been clearly demonstrated. Here, we demonstrate that myosin light chain kinase specifically mediates agonist-induced sarcomere organization during early hypertrophic response. Acute administration of a hypertrophic agonist, phenylephrine, or angiotensin II, causes phosphorylation of myosin light chain 2v both in cultured cardiac myocytes and in the adult heart in vivo. We also show that both sarcomere organization and myosin light chain 2v phosphorylation are dependent on the activation of Ca 2+/calmodulin pathway, a known activator of myosin light chain kinase. These results define a new and specific role of myosin light chain kinase in cardiac myocytes, which may provide a rapid adaptive mechanism in response to hypertrophic stimuli.