Free radicals (FR) play important roles in numerous biological processes, in which their actual contribution can be demonstrated by means of electron paramagnetic resonance (EPR) spectroscopy. Mouse liver is a source of EPR signals originating among various paramagnetic species, such as semiquinones, Fe-S clusters and others. Detailed EPR studies of the normal and stressed liver tissues revealed a difference in the FR signal pattern and was further supported by (immuno)histological and ultrasonography (USG) analyses. EPR: 9-week-old C57BL6/J male mice (25-30g) were randomized to the control (ctrl, n=7) or experimental groups: lipopolysaccharide (LPS, n=7) or red light and photoactive drug – Zinc pheophorbide (Zn-Pheide) (PDT, n=8) stressed. Experimental animals: PDT mice were given I.V. Zn-Pheide (2mg kg-1), and after 70min the injection were anaesthetized I.P. with a mixture of ketamine (30mg kg-1) and xylazine (8mg kg-1). Mice were shaved (1 cm2 of left thoracic area) and exposed for 20min to red light (655nm, 100mW/cm2, 120J/cm2) (Jakubowska, 2013); LPS mice were given I.P. endotoxin (10mg kg-1). Ctrl, PDT and LPS mice were subjected to the administration of spin trap (Ziaja, 2011): I.P. injection of sodium diethyldithiocarbamate (DETC, 500mg kg-1) followed by S.C. injection of iron II sulphate (50mg kg-1) in sodium citrate (250 mg kg-1) 0.5h before the euthanasia (71.5, 71.5 and 5.5h after the treatment, respectively). Livers were collected for EPR and histological analyses. USG: animals (ctrl, PDT, LPS, n=3 in each group) were treated as described above. Mice were anaesthetized with isoflurane (0.5l min-1) 72, 72 and 6h after the treatment, respectively, and VEVO 3D USG Doppler liver signals were recorded; then animals were euthanized and livers were collected for immunohistochemical (CD31, Gr-1, Mac-3) analysis. USG images were analyzed using OsiriX, Python Image Library, VolView , Paraview, Mesh Lab and Blender software. Results: EPR spin trapping studies revealed significant differences in FR liver pattern between the control and experimental animals: in PDT livers white necrotic areas of markedly (~100%) decreased Fe-S cluster signals had developed. LPS livers were a source of intensive signals of NO complexes, in comparison to ctrl. (Immuno)histological analysis indicated characteristics of ischaemic necrosis in PDT livers, but did not reveal differences between ctrl and LPS livers. USG studies allowed for 3D reconstruction of liver vasculature, leaving a clear message of the significant destruction in PDT ischaemic necrotic livers. Conclusions: photodynamic treatment provides a novel, affordable and safe animal model to study liver stress, leading to quick development of ischaemic tissue necrosis. All the animal experiments were approved (No. 46/2012 and 60-62/2013) by the Ethical Committee for Animal Experimentation in Kraków, Poland.