The interaction of light with a photosensitizer, accumulated in a tissue in the presence of oxygen, leads to formation of reactive oxygen species, mainly of singlet oxygen and free radicals. These factors react with biomolecules producing their oxidized states. Reactive oxygen species, such as singlet oxygen and free radicals are able to damage membranes, DNA, enzymes, structural peptides and other cellular structures leading to cell death. An antioxidant protection of cell is formed by enzymes belonging to the family of oxidoreductases: superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione reductase (GR). Photodynamic therapy leads to the increased production of oxidizing toxic forms. It is important to analyze impact of PDT on the activity of antioxidant enzymes, such as SOD, CAT, GPx. The activity of antioxidant enzymes during the photodynamic effect is influenced by both the light energy dose and the concentration of photosensitizer. The presence only of the photosensitizer or only the light energy may also result in changes in the activity of these enzymes. The differences in changes in the activity of these enzymes depend on the type of used photosensitizer. A phenomenon of selective accumulation of photosensitizer in tumor tissues is used in the photodynamic method of tumor diagnosis and treatment.