Both insulin and epidermal growth factor (EGF) significantly increased the activation of p21Ras in Rat-1 fibroblasts transfected with human insulin receptors. As both growth factors have been shown to stimulate guanine nucleotide exchange on p21Ras in permeabilized cells, we examined their effects on total cellular guanine nucleotide exchange factor (GEF) activity in a direct assay, using the rate of dissociation of [3H]GDP from p21Ras as a measure of GEF activity. Insulin increased GEF activity in a time-dependent manner, whereas EGF had no effect on GEF activity at any time point studied. To assess whether EGF stimulates guanine nucleotide exchange by recruiting Grb-2-Sos complexes to the plasma membrane, we measured GEF activity in both the cytosol and the plasma membrane fractions of the insulin- and EGF-treated cells. Insulin increased GEF activity in both fractions (48 +/- 12% [3H]GDP released vs. 24 +/- 6% in control plasma membranes, and 65 +/- 13% vs. 13 +/- 4% in control cytosolic fractions), whereas EGF enhanced only the plasma membrane-associated activity (43 +/- 12% of [3H]GDP release in the plasma membrane fraction and 10 +/- 2% in the cytosol). Western blotting of the subcellular fractions with Grb-2 and Sos antibodies revealed translocation of these elements to the plasma membrane after stimulation of cells with either insulin or EGF. Thus, whereas insulin stimulates guanine nucleotide exchange on Ras by both translocating Grb-2-Sos complexes to the plasma membrane and increasing the GEF activity of Sos, EGF does so by causing a translocation of Grb-2-Sos without increasing GEF activity.