Tyrosine kinases activated by G protein-coupled receptors can phosphorylate and thereby suppress the activity of the delayed rectifier potassium channel Kv1.2. Using a yeast two-hybrid screen, we identified the small GTP-binding protein RhoA as a necessary component in this process. Coimmunoprecipitation experiments confirmed that RhoA associates with Kv1.2. Electrophysiological analyses revealed that overexpression of RhoA markedly reduced the basal current generated by Kv1.2 expressed in Xenopus oocytes. Furthermore, in 293 cells expressing Kv1.2 and ml muscarinic acetylcholine receptors, inactivating RhoA using C3 exoenzyme blocked the ability of ml receptors to suppress Kv1.2 current. Therefore, these results demonstrate that RhoA regulates Kv1.2 activity and is a central component in the mechanism of receptor-mediated tyrosine kinase-dependent suppression of Kv1.2.