Consistent with our previous studies, we continue to evaluate the antioxidant potential of representatives belonging to the Papaveraceae family collected from the northern part of Poland using a muscle tissue model of rainbow trout (Oncorhynchus mykiss Walbaum). Therefore, in the present study, oxidative stress biomarkers [2-thiobarbituric acid reactive substances (TBARS), protein oxidative modification carbonyl derivative content, total antioxidant capacity (TAC)] were used to evaluate the antioxidant activity of stalk and root extracts of Chelidonium majus L. (CM) at doses of 5 mg.mL -1 , 2.5 mg.mL -1 , 1.25 mg.mL -1 , and 0.63 mg.mL -1 . Muscle tissue homogenates of rainbow trout were used in this study. Phosphate buffer was used as a positive control. After incubation of the mixture at 25 °C for 2 h with continuous mixing, samples were used for biochemical studies. Results of our study revealed that a dose of CM extracts of 0.63 mg.mL -1 showed the highest antioxidant activity in the muscle tissue of rainbow trout. The extracts derived mainly from the roots of CM collected from rural areas were effective in reducing the levels of oxidative stress biomarkers by reducing lipid peroxidation markers, which may suggest that the active substances such as alkaloids (chelidonine, sanguinarine, berberine), flavonoids, phenols in these plants can effectively protect the membrane structures in muscle cells of salmonids. We also observed statistically significant reductions in levels of both aldehydic and ketonic derivatives of oxidatively modified proteins in muscle tissue of rainbow trout after incubation with CM extracts at this dose compared to the controls. The comparison of these results shows that CM extracts can effectively inhibit protein damage by scavenging free radicals and/or activation of antioxidant defenses. The secondary metabolites of CM, i.e. polyphenols and alkaloids, are most likely responsible for this effect. Using doses of 5 mg.mL -1 , 2.5 mg.mL -1 , and 1.25 mg.mL -1 of both root and stalk extracts in vitro study, statistically significant increases in levels of TBARS and OMP were observed. Screening of species belonging to the family Papaveraceae for other biological activities, including antioxidant activity, is essential and may be effective in the search for preventive measures in the pathogenesis of diseases caused by oxidative stress in human and veterinary medicine.