Insulin increases activity of the guanine nucleotide exchange factor (GEF) in Rat-1 fibroblasts transfected with human insulin receptors (HIRc cells), thereby promoting formation of the active form of p21Ras (p21Ras•GTP). In order to identify the upstream molecules mediating this aspect of insulin action, we selectively removed some of these molecules by immunoprecipitation and examined GEF activity in the post-immunoprecipitation lysated of the insulin-treated HIRc cells. The removal of Shc or Grb-2 depleted GEF activity from the cell lysates, whereas immuno-precipitation of the insulin receptors, IRS-1, PLCγ and GAP, were without effect. In summary, the current data demonstrate that a majority of cellular Ras GEF activity after insulin stimulation is associated with Shc and involves interactions among Shc, Grb-2 and Sos.