The purpose of this study was to assess the potential effects of Rhus coriaria L. (sumac) and of Cinnamomum zeylanicum L. bark on the selected serum cytokines as possible serum tumor markers - interleukin-6 (IL-6), interleukin-10 (IL-10), and tumor necrosis factor-α (TNF-α) in the rat model of mammary carcinogenesis. R. coriaria and C. zeylanicum bark were used as the chemopreventive-therapeutic agents taken by rats in the powder form in the diet at two different concentrations during the entire period of two experiments carried out separately: lower concentration 1 g/kg - 0.1% and higher concentration 10 g/kg - 1%. The serum levels of cytokines of IL-6, IL-10, and TNF-α were determined using an enzyme-linked immunosorbent assay. In the first experiment treated with R. coriaria, a significant decrease in serum levels of IL-6 and TNF-α was present at higher concentrations compared to the chemoprevention-free control group. R. coriaria at lower concentrations non-significantly reduced the serum levels of IL-6 and TNF-α when compared to controls. A significant decrease in serum levels of TNF-α was present at higher concentrations compared to lower concentrations. The significant effect of R. coriaria on the serum levels of IL-10 was not observed. In the second experiment treated with C. zeylanicum bark, a significant decrease in serum levels of IL-6 was observed in lower and higher concentrations compared to the chemoprevention-free control group. C. zeylanicum bark non-significantly reduced the serum levels of TNF-α and had no effect on the serum levels of IL-10. In conclusion, R. coriaria and C. zeylanicum bark demonstrated significant anti-inflammatory effects by analyzing the selected serum cytokine levels in the rat breast carcinoma model. Observed anti-inflammatory effects of both plant-natural substances were associated with their anticancer activities in rats.