Generation of reactive oxygen species (ROS) by Ras oncogene-induced NADPH oxidase (Nox) 1 is required for Ras transformation phenotypes including anchorage-independent growth, morphological transformation, and tumorigenesity, but the signaling mechanism downstream of Nox1 remains elusive. Rho is known to be a critical regulator of actin stress fiber formation. Nonetheless, Rho was reported to no longer couple to loss of actin stress fibers in Ras-transformed Swiss3T3 cells despite the elevation of Rho activity. In this study, however, we demonstrate that Rho is inactivated in K-Ras-transformed normal rat kidney cells, and that abrogation of Nox1-generated ROS by Nox1 small interference RNAs or diphenyleneiodonium restores Rho activation, suggesting that Nox1-generated oxidants mediate down-regulation of the Rho activity. This down-regulation involves oxidative inactivation of the low molecular weight protein-tyrosine phosphatase by Nox1-generated ROS and a subsequent elevation in the tyrosine-phosphorylated active form of p190RhoGAP, the direct target of the phosphatase. Furthermore, the decreased Rho activity leads to disruption of both actin stress fibers and focal adhesions in Ras-transformed cells. As for Rac1, Rac1 also appears to participate in the down-regulation of Rho via Nox1. Our discovery defines a mediating role of Nox1-redox signaling for Ras oncogene-induced actin cytoskeletal changes.