The effects of GSM (global system for mobile communication) exposure on the genomic response in rat brains were studied using a special GSM radiofrequency test set, connected to a commercial cellular phone operating in the discontinuous transmission mode. Rats were restrained in a carrousel of circularly arranged plastic tubes and sham-exposed or irradiated for a duration of 4 h at specific brain absorption rates (SAR) of 0.3 W/kg, 1.5 W/kg and 7.5 W/kg, respectively. Changes in the mRNAs of hsp70, the immediate early genes (lEGs) c-fos and c-jull and the glial structure gene GFAP were measured by in situ hybridization. Protein products of lEGs, stress proteins and marker proteins of astroglial and microglial activation were assessed by immunocytochemistry. Cell proliferation was evaluated by bromodeoxyuridine incorporation. Effects on the permeability of the blood-brain barrier were investigated immunocytochemically by demonstrating extravasation of serum albumin. Immediately after GSM exposure, semiquantitative evaluation of ill situ hybridization exhibited a slight induction of hsp70 mRNA in the cerebellum and hippocampus after SAR 7.5 W/kg, but not at lower intensities. A slightly increased expression of c-fos mRNA was observed in the cerebellum, neocortex and piriform cortex of all groups subjected to immobilization, but no upregulation was found among different exposure conditions. c-jun and GFAP mRNA did not increase in any of the experimental groups. 24 h after exposure, immunocytochemical analysis of FOS and JUN proteins or of KROX-20 and -24 was unaltered. Seven days after exposure, neither increased cell proliferation nor altered expression of astroglial and microglial marker proteins were observed. The increase in serum albumin extravasation reached signifiance only in the group at highest SAR of 7.5 W/kg immediately after exposure. In summary, acute high intensity microwave exposure of immobilized rats may induce some minor stress response and minor but reversible increases of blood-brain barrier permeability. However, microwave exposure in the frequency and intensity range of mobile telephones does not result in lasting adaptive or reactive changes of gene expression and is unlikely to produce pathologically significant changes of the blood-brain barrier.