Ras proteins are signal transducers for many cellular responses. However, it is not well established whether Rassignaling also contributes to apoptosis. We have constructed H-Ras R12 -transformed Rat1 fibroblasts using tetracycline operator/repressor (TetO/TetR)-based conditional vectors. Rat1/TetO-Ras R12 (Rat1-Ras) cells produced high levels of HRas R12 protein and exhibited oncogenic transformation. Treatment of Rat1-Ras cells with 0.1% serum triggered massive apoptosis. Rat1-Ras cells expressed increased basal activities of extracellular response kinase (ERK) and p46/p54 stress-activated protein kinase/c-Jun NH 2 -terminal kinase (SAPK/JNK). Interestingly, Ras-dependent apoptosis correlated with further persistent activation of both p46 and p54 SAPK/JNK and concurrent inhibition of ERK. Differential modulation of SAPK/JNK and ERK was not detected in tetracycline-treated cells that did not commit apoptosis. Furthermore, two Bcl-x related proteins of 15 kDa and 18 kDa were highly induced in apoptotic Rat1-Ras cells. Our results establish a direct role for Ras in apoptosis, and suggest a functional relationship between H-Ras, SAPK/JNK, ERK and Bcl-x in regulating apoptosis.