**Abstract**

The pathogenesis of rheumatoid arthritis is poorly understood; however, elevated oxidative stress has been described to be involved. In this chapter, we present experiments with endogenous molecules bearing antioxidative properties. In our studies, we used male Lewis rats, and the arthritis was induced with *Mycobacterium butyricum*. In the first experiment, we tested coenzyme Q10 (CoQ 10) in the oral daily dose of 100 mg/kg b.w. Markers of inflammation and total antioxidant status were corrected in the group supplemented. CoQ10 treatment significantly improved concentrations of the investigated endogenous antioxidants. Further as an important fact, we consider a good bioavailability of used CoQ10 formulation which was confirmed by increased CoQ concentrations in plasma, tissue, and mitochondria from skeletal muscles. In the second study, we describe the results with hyaluronic acid (HA) administered in oral daily doses of 0.5 mg and 5 mg/kg b.w. and of different molecular weights (0.43, 0.99, and 1.73 MDa). A notable antioxidative effect of HA was assessed: its administration increased the activities of antioxidant enzymes (superoxide dismutase and glutathione peroxidase) in erythrocytes and total antioxidant capacity of plasma and reduced the marker of oxidative damage to lipids—plasmatic lipid hydroperoxides. HA with the highest molecular weight showed the most significant effect.

**Keywords:** oxidative stress, antioxidants, coenzyme Q10, hyaluronan, arthritis
