The potential health consequences of common exposure of human organisms to high-voltage industrial frequency and radio-frequency electromagnetic fields have not yet been completely recognized. The aim of this study was to evaluate the impact of a 50 Hz electromagnetic field generated by high-voltage alternating electric current transmission lines, a 900 MHz electromagnetic field emitted by mobile phones, and simultaneous exposure to those forms of electromagnetic fields on prooxidant and antioxidant processes in the liver tissue of 40 male rats randomly divided into three experimental groups exposed to different forms of electromagnetic fields and a control sham-exposed one. After completing 28 daily exposures in obtained liver tissue homogenates we determined the contents of markers of prooxidant processes (total oxidant capacity and malone dialdehyde), as well as activity of selected antioxidant enzymes (superoxide dismutase, its isoenzymes Cu,Zn-SOD and Mn-SOD, catalase and glutathione peroxidase). Moreover, cell ultrastructures in specimens from liver samples were examined under an electron microscope. A four-week exposure to a high-voltage 50 Hz electromagnetic field did not affect the prooxidant/antioxidant balance in the liver tissue of rats, while exposure to the radiofrequency electromagnetic field emitted by a mobile phone and simultaneous exposure to both forms of electromagnetic field significantly inhibited the intensity of prooxidant processes and decreased the activity of antioxidant enzymes in liver tissue, especially in the case of simultaneous action of both fields – probably due to their excessive use for restoring alternatprooxidant/ antioxidant balance without causing any structural changes in liver cells visible in an electron microscope.