Insulin and insulin-like growth factors (IGF-I and IGF-II) support the survival and differentiation of many types of neurons, including those from fetal chick forebrain. The mechanisms by which these peptides exert their neurotrophic actions are poorly understood. The aims of this study were to determine if insulin and IGF-I activate p21ras in fetal chick forebrain neurons and if activation of p21ras mediates the neurotrophic actions of these peptides. Activation of neuronal p21ras was examined by measuring the amount of GTP bound to p21ras before and after growth factor treatment. Insulin and IGF-I increased the ratio of GTP/GTP + GDP by 31 +/- 9.0% and 36 +/- 8.0%, respectively, p21Ras activation by insulin and IGF-I was maximal within 5 min. In the presence of insulin the response was sustained out to 180 min, whereas the response to IGF-I decreased significantly by 180 min. Both peptides stimulated p21ras at low concentrations with a maximal response obtained at 10 ng/ml for each peptide, idicating that insulin and IGF-I activate ras by interacting with their homologous receptor. Pretreatment of neurons with lovastatin (2 micrograms/ml), an inhibitor of ras isoprenylation, completely blocked the activation of p21ras by insulin and IGF-I. Lovastatin also blocked the ability of these growth factors to support the survival and differentiation of fetal chick neurons in culture. We conclude that insulin and IGF-I activate p21ras in fetal chick forebrain neurons by increasing the amount of GTP bound to p21ras. The activation of neuronal p21ras is necessary for insulin and IGF-I to promote survival and differentiation in these neurons.