In chemotherapy practice the dexamethasone is used as concomitant agent to mitigate the side effects of antitumor drug cisplatin. However, it is known, that both cisplatin and dexamethasone are capable to stimulate the production of reactive oxygen spacies, which targets various biomolecules, including lipids. This work is devoted to the study of quantitative changes in products of lipid peroxidation in the nuclei of cells of some tissues of rats after separate and combined use of cisplatin and dexamethasone. The amount of lipid peroxidation products was determined by spectrophotometric method after its extraction by heptan-isopropanol mixture. When used separately, cisplatin and dexamethasone enhance the formation of lipid peroxidation products in the studied tissues of rats to various degrees. As a result of these alterations, the values of the oxidation index in the studied nuclear preparations also changed accordingly. During the combined use of cisplatin and dexamethasone there is some antagonism in the action of these drugs. Instead of the supposed summation of the effects of enhancing lipid peroxidation processes, “masking” of the effect of cisplatin with dexamethasone is observed. Thus, it has been assumed that exactly such antagonism revealed during combined action helps mitigate the side effects of cisplatin with dexamethasone.