Oxidative stress is a state of an altered redox balance caused by unequal production and depletion of reactive molecular species. In physiological conditions, reactive oxygen species (ROS) serve as important signaling molecules and are involved in cell viability. When their concentration is heightened, they are considered to be the main drivers of the aging process due to their ability to disrupt the integrity of all macromolecules. Senescence or aging is a state in which a cell progressively falls into metabolic and physiological disbalance caused by extensive damage to its components. A cell usually enters senescence due to irreparable DNA damage. If the damage is not extensive, the cell is subjected to repair. Otherwise, the damage will lead to the activation of mechanisms involved in cell death. The effect of ROS molecules on proteins is observed through the sulfur-containing amino acid methionine (Met, M), which is readily oxidized in said conditions. Methionine sulfoxide (MetO) is present in two epimers - S and R. The two epimers are reduced by enzymes, part of the methionine sulfoxide reductase (Msr) family, Msr A and Msr B. Because of stereospecificity, MsrA reduces the S-epimer while MsrB reduces the R-epimer of MetO. The main goal of this master's thesis was to elucidate the existence of possible sex and age differences consistent with the changes found in methalotionein expression. Methalotionein expression is dominant in females, heightens with age and is potentially involved in MsrB3 regeneration. On the basis of MsrB3 expression in rat kidneys, we produced the target mRNA using the RT-PCR method, established the presence of proteins in tissue homogenates with western blot analysis and the overall distribution of proteins in tissue samples via imunnofluorescent staining. Borderline positive differences were confirmed in females following the nucleic acid analysis and western blot analysis, but the immunofluorescent staining produced inconsistent results with intense vacuolar staining in males, so antibody specificity has to be reviewed.. The final results were contrary to the ones expected and at present do not confirm the correlation between metalothionein and MsrB3 expression in the kidneys.