Abstract Amplification and/or mutations of the epidermal growth factor (EGF) receptor have been frequently reported in human malignant gliomas, the most common primary tumor of the adult central nervous system. We have analyzed a panel of established human glioma cell lines for EGF receptor expression. The EGF receptor was expressed in all of the glioma cell lines tested, with highest levels found in the cell line U343MG-a. In addition, various amounts of a truncated form of the EGF receptor were detected. The platelet-derived growth factor (PDGF) α receptor, analyzed for comparison, was expressed at low levels in human glioma cells, with the exception of U-118MG and U-373MG cells. The truncated form of the EGF receptor has been discussed as a constitutively active variant of the receptor. Using antibodies directed against the active form of the EGF receptor, we show here that the truncated variant of the EGF receptor in U343MG-a cells is not in the active conformation. However, the full-length EGF receptor, highly expressed in U343MG-a cells, was very rapidly activated following EGF treatment. In line with this, phosphorylation and activation of the mitogen-activated protein kinase/extracellular signal-regulated protein kinase (ERK) in U343MG-a cells required administration of EGF. Moreover, using highly specific riboprobes we observed that EGF signaling increased the Egr-1 mRNA concentration in human glioma cells within 30 min. The increase in the Egr-1 mRNA concentration was followed by a transient synthesis of the Egr-1 protein. Likewise, Egr-1 mRNA and protein concentrations were increased in U-118MG and U-373MG cells treated with PDGF. The synthesis of Egr-1 in human glioma cells as a result of EGF or PDGF stimulation indicates that Egr-1 may be an important “late” part of the EGF and PDGF-initiated signaling cascades suggesting that Egr-1 functions as a “third messenger” in glioma cells connecting growth factor stimulation with changes in gene transcription.