Living cells exposed to changes in the surrounding oxygen tension, have the ability to adapt to the new environment through the regulatory effect of intracellular mediators. In an effort to identify important proteins that may be involved in the hyperoxic response, we performed proteomic analysis on the human choriocarcinoma cell line JEG-3, incubated under high oxygen tension (carbogen, 95% O2/5% CO(2)) or air (21% oxygen/5% CO(2)). We identified 13 protein spots that were significantly down-regulated (p < 0.05) in JEG-3 cells incubated under hyperoxic conditions compared to standard conditions. Ten of these spots were positively identified by matrix-assisted laser desorption/ionization time of flight-mass spectrometry as nine different proteins: Villin 2, tublin beta, profilin I, glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate mutase, peroxiredoxin 1, neuroplypeptide h3, poly(rC)-binding protein 1 and cyclophilin A. These proteins have been implicated in regulating cytoskeletal structure, glycolysis, redox status, signal transduction, transcription and protein folding. The data obtained are consistent with the roles of these proteins in mediating cellular response to oxidative stress and in regulating cell proliferation and motility.