Growth hormone (GH) plays an important role in growth and metabolism by signaling via at least three major pathways, including STATs, ERK1/2, and phosphatidylinositol 3-kinase/Akt. Physiological concentrations of insulin promote growth probably by modulating liver GH receptor (GHR) levels in vivo, but the possible effects of insulin on GH-induced post-GHR signaling have yet to be studied. We hypothesized that short-term insulin, similar to the fluctuations that occur following feeding, affects GH-induced post-GHR signaling. Our present studies suggest that, in rat H4IIE hepatoma cells, insulin (4 h or less) selectively enhanced GH-induced phosphorylation of MEK1/2 and ERK1/2, but not GH-induced activation of STAT5 and Akt. Although insulin pretreatment altered GH-induced formation of Shc·Grb2·SOS complex, it did not significantly affect GH-induced activation of other signaling intermediates upstream of MEK/ERK, including JAK2, Ras, and Raf-1. Immunofluorescent staining indicated that insulin pretreatment facilitated GH-induced cell membrane translocation of MEK1/2. Insulin pretreatment also increased the amount of MEK association with its scaffolding protein, KSR. In summary, short-term insulin treatment of cultured, liver-derived cells selectively sensitized GH-induced MEK/ERK phosphorylation independent of JAK2, Ras, and Raf-1, but likely resulted from increased cell membrane translocation of MEK1/2. These findings suggest that insulin may be necessary for sensitization of cells to GH-induced ERK1/2 activation and provides a potential cellular mechanism by which insulin promotes growth.