In the present study, to examine the dynamic changes in the localization of nuclear ER induced by growth factors, we used time-lapse confocal microscopy to directly visualized ER fused with green fluorescent protein (GFP-ER) in single living cells treated with epidermal growth factor (EGF) or insulin growth factor-1 (IGF-1). We observed that 17-estradiol (E2) changed the normally diffuse distribution of GFP-ER throughout the nucleoplasm to a hyperspeckled distribution within 10 min. Both EGF and IGF-1 also changed the nuclear distribution of GFP-ER, similarly to E2 treatment. However, the time courses of the nuclear redistribution of GFP-ER induced by EGF or IGF-1 were different from that induced by E2 treatment. In the EGF-treated cells, the GFP-ER nuclear redistribution was observed at 30 min and reached a maximum at 60 min, while in the IGF-1-treated cells, the GFP-ER nuclear redistribution was observed at 60 min and reached a maximum at 90 min. The EGF-induced redistribution of GFP-ER was blocked by pretreatment with a mitogen-activated protein (MAP) kinase cascade inhibitor, PD98059, while the IGF-1-induced redistribution of GFP-ER was blocked by pretreatment with a phosphatidylinositol (PI) 3 kinase inhibitor, LY294002. Analysis using an AF-2 domain deletion mutant of GFP-ER showed that the change in the distribution of GFP-ER was not induced by E2, EGF or IGF-1 treatment. These data suggest that MAP kinase and PI3 kinase cascades are involved in the nuclear redistribution of ER by EGF and IGF-1, respectively, and that the AF-2 domain of ER may be needed for the nuclear redistribution of ER.