In 3T3-L1 fibroblasts, Ras proteins mediate both insulin-induced differentiation to adipocytes and its activation of cytosolic serine/threonine kinases, including Raf-1 kinase, mitogen-activated protein kinase (MAPK), and Rsk. Here, we report that insulin- and Ras-induced activation of MAPK is not required for the differentiation process and in fact antagonizes it. The treatment of 3T3-L1 preadipocytes with MEK-specific inhibitor PD98059 blocked insulin- and Ras-induced MAPK activation but had no effect on or slightly enhanced adipocytic differentiation. Tumor necrosis factor alpha (TNF-alpha), an inhibitor of insulin-stimulated adipogenesis, activated MAPK in 3T3-L1 cells. PD98059 treatment blocked MAPK activation by TNF-alpha and reversed the blockade of adipogenesis mediated by low (1 ng/ml) TNF-alpha concentrations. 3T3-L1 transfectants containing hyperactivated MEK1 or overexpressed MAPK displayed impaired adipocytic differentiation. PD98059 treatment also reversed the blockade of differentiation in MEK1 transfectants. These results indicate that MAPK does not promote but can contribute to inhibition of the process of adipocytic differentiation of 3T3-L1 cells.