Porphyrins are a group of organic compounds, which have been increasingly used as photosensitizers (PS) in photodynamic therapy (PDT) in recent years. PDT is based on the mechanism of phototoxic excitation of PS with the help of light of appropriate wavelength, thus producing reactive oxygen species responsible for cytotoxicity. In preclinical studies using the in vitro and in vivo approaches, it is necessary to demonstrate the phototoxicity of PS only in the presence of excitation wavelength, as well as non-toxicity in the absence of light. This thesis is a continuation of the (photo)toxicity study of newly synthesized N-oxidized and N-methylated (3-pyridyl)porphyrins, conducted in vitro on human cancer cells. The aim of this study was to develop a protocol for testing the stability and toxicity of N-oxidized and N-methylated (3-pyridyl) porphyrins in an in vivo model of D. melanogaster, using TMPyP3. Spectroscopic methods confirmed the chemical stability of TMPyP3 in methanol, distilled water, PBS and solutions of proteins and physiological cations. TMPyP3 was found to remain chemically stable only in nutrient medium for D. melanogaster without yeast addition. The concentration of the absorbed compound is about 1 nM, and is higher in pre-starved flies fed for 180 minutes with 40 μM TMPyP3 compared to non-starved flies fed for 24 hours. TMPyP3 showed no acute dark toxicity for D. melanogaster. It has been shown that the PDT therapy using LED red light source at an intensity of 40 mW/cm2 is not lethal, but is potentially neurotoxic to D. melanogaster fed with 40 μM TMPyP3 for 180 minutes regardless of the duration of illumination. The protocol examined in this study proved effective for detecting the phototoxicity of PDT therapy with 3-pyridylporphyrins in an in vivo model of D. melanogaster. With further development of the protocol, D. melanogaster could become an important model in preclinical studies of 3-pyridylporphyrins, enabling rapid and cost-effective toxicity testing of a large number of these compounds, after which only the best candidates will access classical studies in mammalian models.