Vascular smooth muscle cell (VSMC) proliferation is a key event in the progression of atherosclerosis. Induction of both c-fos (through the transcription factor Elk-1) and c-jun, both immediate early genes, is important for the stimulation of VSMC proliferation and migration. It was earlier found that p38 mitogen-activated protein (MAP) kinase upregulates c-jun gene transcription through phosphorylation of two myocyte enhancer factor 2 (MEF2) family transcription factors, MEF2A and MEF2C, while big MAP kinase 1 (BMK1) may upregulate c-jun gene transcription through MEF2A, MEF2C, and also MEF2D. Here, we report that inhibition of BMK1 by a dominant negative form of MEK5 or pharmacologic inhibition of p38 by SB 203580 additively suppress serum-induced VSMC proliferation. This additive effect of p38 and BMK1 inhibition implies that these two kinases coordinately regulate MEF2 transcription factors. The exclusive activation of MEF2D by BMK1 appears required for this cooperative upregulation of c-jun in VSMC, and coactivation of p38 and BMK1 also has additive effects on the activation of a reporter gene linked to the c-jun promoter in our experimental system. Thus, coordinate activity of both the p38 and BMK1 pathways appears necessary for optimal transcription of c-jun and, pari pasu, VSMC proliferation. These results may have implications for the future design of pharmacologic agents for inhibition of VSMC growth.