Ras plays a key role in regulating cellular proliferation, differentiation, and transformation. Raf is the major effector of Ras in the Ras > Raf > Mek > extracellular signal-activated kinase (ERK) cascade. A second effector is phosphoinositide 3-OH kinase (PI 3-kinase), which, in turn, activates the small G protein Rac. Rac also has multiple effectors, one of which is the serine threonine kinase Pak (p65(Pak)). Here we show that Ras, but not Raf, activates Pak1 in cotransfection assays of Rat-1 cells but not NIH 3T3 cells. We tested agents that activate or block specific components downstream of Ras and demonstrate a Ras > PI 3-kinase > Rac/Cdc42 > Pak signal. Although these studies suggest that the signal from Ras through PI 3-kinase is sufficient to activate Pak, additional studies suggested that other effectors contribute to Pak activation. RasV12S35 and RasV12G37, two effector mutant proteins which fail to activate PI 3-kinase, did not activate Pak when tested alone but activated Pak when they were cotransfected. Similarly, RacV12H40, an effector mutant that does not bind Pak, and Rho both cooperated with Raf to activate Pak. A dominant negative Rho mutant also inhibited Ras activation of Pak. All combinations of Rac/Raf and Ras/Raf and Rho/Raf effector mutants that transform cells cooperatively stimulated ERK. Cooperation was Pak dependent, since all combinations were inhibited by kinase-deficient Pak mutants in both transformation assays and ERK activation assays. These data suggest that other Ras effectors can collaborate with PI 3-kinase and with each other to activate Pak. Furthermore, the strong correlation between Pak activation and cooperative transformation suggests that Pak activation is necessary, although not sufficient, for cooperative transformation of Rat-1 fibroblasts by Ras, Rac, and Rho.