IGF-1 is a potent mediator of both PA proliferation and differentiation. Switching from the former to latter effects requires PA growth arrest. The mitogen activated protein kinases (MAPKs) are essential to mitogenic signaling via many growth factor receptors. To study modulation of IGF-1 signaling during PA differentiation, we examined the activation of MAPKs by IGF-1 in proliferating, growth-arrested and differentiating 3T3-L1 PAs. MAPK activity was measured as 32P-incorporation into myelin basic protein (MBP). Proliferating PAs showed maximal IGF-1-stimulation of MAPK which was 4-fold above baseline and similar to that seen with EGF, a potent 3T3-L1 mitogen. Growth-arrested preadipocytes showed consistently less potent IGF-1 effect, with a maximal stimulation 3-fold over baseline and half the EGF response. Early differentiating adipocytes' maximal IGF-1 activation was 2-fold over baseline but was only one third the response to EGF. Immunoprecipitation of MAPK confirmed specificity of MBP phosphorylation. In all cases, MAPK activation was maximal at 5 minutes. Dose-response curves in proliferating cells revealed maximal stimulation at only 10 ng/ml IGF-1 versus 10 μg/ml insulin. Interestingly, MAPK activation by IGF-1 was greater in the presence of conditioned versus fresh serum-free medium, suggesting potentiation of signaling by IGF binding proteins. We conclude that PA differentiation is accompanied by a selective uncoupling of IGF-1 signaling via MAPK with no change in EGF signaling. This may indicate that the process of adipogenesis encompasses a switch in IGF-1 signal transduction.