Relevance. In this article, we present experimental study on the effect of photodynamic therapy with a chlorin-type photosensitizer on the hematological and biochemical blood parameters of cats with malignant oncological diseases. Methods. During conducting studies in experimental animals, we took blood samples from 44 cats aged 8 to 16 years, of different breeds and different sexes, before photodynamic therapy and two days after treatment, in order to determine the possible effect of therapy on the blood counts of patients. Blood sampling was carried out according to the standard method. Hematological examination was performed on a PCE90vet analyzer (HTI, USA), biochemical on an automatic biochemical analyzer Biosystems A15 (Biosystems, Spain) and on a semi-automatic analyzer Biohaem SA (HTI, USA). Photodynamic therapy was performed according to the standard method, with the preliminary introduction of the photosensitizer “Photoditazine” at a dose of 0.8– 1 mg/kg, 3 hours before irradiation. Results. The most significant changes in the hematological study were found among the following parameters: the level of white blood cells after irradiation increased by 18%, the number of eosinophils decreased by 28%, the number of segmented neutrophils increased by 11%, the content of lymphocytes decreased by 21%. When studying the biochemical parameters of blood, changes were found in the following parameter: the glucose level increased by 13% after irradiation, the level of GGT decreased by 19% after irradiation. At the same time, all indicators of the hematological and biochemical composition of the blood were within the normal values for this type of animal. Photodynamic therapy does not significantly affect the hematological and biochemical parameters of the blood of cats, this proves the safety of using this method of treatment in these animals.