The phosphorylation states of three proteins implicated in the action of insulin on translation were investigated, i.e., 70-kDa ribosomal protein S6 kinase (p70S6k), eukaryotic initiation factor (eIF) 4E, and the eIF-4E binding protein 4E-BP1. Addition of insulin caused a stimulation of protein synthesis in L6 myoblasts in culture, an effect that was blocked by inhibitors of phosphatidylinositide-3-OH kinase (wortmannin), p70S6k (rapamycin), and mitogen-activated protein kinase (MAP kinase) kinase (PD-98059). The stimulation of protein synthesis was accompanied by increased phosphorylation of p70S6k, an effect that was blocked by rapamycin and wortmannin but not PD-98059. Insulin caused dephosphorylation of eIF-4E, an effect that appeared to be mediated by the p70S6k pathway. Insulin also stimulated phosphorylation of 4E-BP1 as well as dissociation of the 4E-BP1.eIF-4E complex. Both rapamycin and wortmannin completely blocked the insulin-induced changes in 4E-BP1 phosphorylation and association of 4E-BP1 and eIF-4E; PD-98059 had no effect on either parameter. Finally, insulin stimulated formation of the active eIF-4G.eIF-4E complex, an effect that was not prevented by any of the inhibitors. Overall, the results suggest that insulin stimulates protein synthesis in L6 myoblasts in part through utilization of both the p70S6k and MAP kinase signal transduction pathways.