Cells transformed by the oncogenic small GTPase, Ras, display a radioresistant phenotype in response to ionizing radiation (IR). To determine the mechanisms by which Ras mediates radioresistance in epithelial cells, we assessed the importance of three major survival pathways that can be activated by Ras [phosphatidylinositol 3-kinase (PI3-K)>Akt, nuclear factor κB (NF-κB), and Raf>mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK)>extracellular signal-regulated kinase] as necessary or sufficient for Ras-mediated radioresistance in matched pairs of RIE-1 rat intestinal epithelial cells expressing oncogenic Ras or empty vector (RIE-Ras and RIE-vector). Inhibiting PI3-K with LY294002 sensitized RIE-1 cells to IR in a dose-dependent manner, indicating that PI3-K is necessary for radioresistance, whereas inhibition of NF-κB with the super-repressor IκBα had little effect on survival. Expression of either the constitutively active catalytic subunit of PI3-K, p110α-CAAX, or the Ras effector domain mutant 12V/40C, which retains binding to PI3-K but is impaired in binding to other Ras effectors, was sufficient to confer partial radioresistance. Expression of either a constitutively active form of the serine/threonine kinase Raf-1 or the Ras effector domain mutant 12V/35S, which retains binding to Raf but is impaired in binding to other Ras effectors, was also sufficient to confer partial radioresistance. Surprisingly, however, even complete inhibition of MEK activity by using U0126 resulted in no change in post-IR survival whatsoever. Thus, whereas Raf contributes to Ras-mediated radioresistance, this is accomplished through a MEK-independent pathway. Taken together, these results indicate that multiple pathways, including both PI3-K-dependent and Raf-dependent but MEK-independent signaling, are required for Ras-mediated radioresistance in epithelial cells. Finally, we demonstrate that Ras-mediated radioresistance can be uncoupled from Ras-mediated transformation, in that PI3-K is required for radioresistance but not transformation, whereas MEK and NF-κB are required for transformation but not radioresistance in RIE-1 epithelial cells.