p38 MAP kinases (p38) and c-Jun N-terminal protein kinases (JNK) have been associated with TNF-α-induced apoptosis. However, recent studies indicate that an early but brief activation of JNK and/or p38 may actually protect some cells from TNF-α-induced apoptosis. Whether the activation of JNK and p38 provides a pro- or anti-apoptotic signal for TNF-α has been controversial. In this study, we investigated the role of p38 in the regulation of TNF-α cytotoxicity in rat mesangial cells. Treatment of the cells with TNF-α alone had little effect on their viability, but they became very sensitive to apoptosis when treated with TNF-α in the presence of the p38 inhibitor SB 203580. These results suggested that the p38 pathway is critical for mesangial cells to survive the toxic effect of TNF-α. Using adenovirus-mediated gene transfer technique, we further demonstrated that p38β, but not p38α, is essential to protect the cells from TNF-α toxicity. It has been speculated that there is a synergetic interaction between the p38 and the nuclear factor-κB (NF-κB) pathways in protecting certain cells from apoptosis. However, expression of neither p38β nor its dominant negative mutant in mesangial cells interfered with TNF-α-induced translocation of NF-κB, the initial step of NF-κB activation. While it is unclear whether p38β regulates NF-κB transcription activity at other steps, it is apparent that p38β does not affect TNF-α-induced NF-κB activation at the stage of nuclear translocation. J. Cell. Biochem. 82: 556–565, 2001. © 2001 Wiley-Liss, Inc.