Background. Electromagnetic irradiation with extremely high frequencies (EHF EMI) and low intensity affects living organisms of a different level of organization, but the mechanism(s) of its influence is still not understood well. This study was undertaken to examine the effects of EHF EMI on tissue lipid peroxidation (LPO) of whole body exposure rats.Material and methods. Male Wistar rats were selected for this study. The animals were divided into two groups: sham exposed and experimental. The rats were exposed to the EMI of 42.2 and 50.3 GHz frequencies (power density 0.06 mW/cm 2 ) for 20 min/ day, for five days. The content of malondialdehyde (MDA) as the final product of the LPO was estimated in brain, liver, heart, and skeletal muscle of rats.Results. Treatment with EMI induced oxidative stress in different organs of the rats, which was indicated by the changes of MDA level depending on the EMI frequency used and exposure duration. The MDA rate shows significant increase in brain (P < 0.001) depending on the treatment duration for both EMI frequencies used. The slightly elevated levels of MDA in the liver were observed among rats in 50.3 GHz frequency EMI-exposed group. Concerning the skeletal muscle and especially the cardiac tissue, the MDA values remained at the same levels in experimental and control groups and did not differ significantly.Conclusions. The EHF EMI applied in the multiple mode significantly enhanced the lipid peroxidation level in the brain and slightly increased the same parameter in liver. The obtained data indicate possible health implications of such exposures, which may cause damage in brain.