To determine whether insulin activates protein kinase C in BC3H-1 myocytes, we evaluated changes in protein phosphorylation, protein kinase activities, and the intracellular translocation of protein kinase C activity in response to insulin and phorbol esters. Phorbol 12-myristate 13-acetate (PMA), but not insulin, stimulated the phosphorylation of an acidic Mr 80,000 protein which has been shown to be an apparently specific marker for protein kinase C activation. In addition, PMA, but not insulin, stimulated the rapid association of protein kinase C activity with a cellular particulate fraction. In contrast to these differences, both insulin and PMA stimulated the phosphorylation of ribosomal protein S6 and activated a ribosomal protein S6 kinase in cell-free extracts from cells exposed to these agents. In cells exposed to high concentrations of PMA for 16 h, protein kinase C activity and immunoreactivity were abolished, without changes in cellular morphology. Under these conditions, insulin, but not PMA, stimulated phosphorylation of the ribosomal protein S6 in intact cells and activated the S6 kinase in cell-free extracts derived from insulin-treated intact cells. We conclude that: insulin does not appear to activate protein kinase C in BC3H-1 myocytes, at least as assessed by phosphorylation of the Mr 80,000 protein; both insulin and PMA activate an S6 protein kinase in these cells; and insulin can promote S6 phosphorylation and activate the S6 kinase normally in protein kinase C-deficient cells. Activation of the S6 kinase by insulin and PMA, although apparently proceeding through different mechanisms, may explain some of the similar biological actions of these compounds in BC3H-1 myocytes.