Atrogin1/MAFbx is an ubiquitin ligase that mediates muscle atrophy in a variety of catabolic states. We recently found that H2O2 stimulates atrogin1/MAFbx gene expression. Since the cytokine tumor necrosis factor-alpha (TNF-alpha) stimulates both reactive oxygen production and general activity of the ubiquitin conjugating pathway, we hypothesized that TNF-alpha would also increase atrogin1/MAFbx gene expression. As with H2O2, we found that TNF-alpha exposure up-regulates atrogin1/MAFbx mRNA within 2 h in C2C12 myotubes. Intraperitoneal injection of TNF-alpha increased atrogin1/MAFbx mRNA in skeletal muscle of adult mice within 4 h. Exposing myotubes to either TNF-alpha or H2O2 also produced general activation of the mitogen-activated protein kinases (MAPKs): p38, ERK1/2, and JNK. The increase in atrogin1/MAFbx gene expression induced by TNF-alpha was not altered significantly by ERK inhibitor PD98059 or the JNK inhibitor SP600125. In contrast, atrogin1/MAFbx up-regulation and the associated increase in ubiquitin conjugating activity were both blunted by p38 inhibitors, either SB203580 or curcumin. These data suggest that TNF-alpha acts via p38 to increase atrogin1/MAFbx gene expression in skeletal muscle.