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Electrolyte leakage (EL) is the method commonly used to test the cell membrane integrity of plants under stress conditions. The cells of the leaf may be damaged by ozone (O3) entering the intercellular space as an oxidative stress agent. The modified EL method was used to test the oxidative stability (OxS) of plant tissue against O3-induced oxidative stress. The modification includes simulation of the artificial oxidative stress by additional ozonation of plant samples in the laboratory chamber. This modified EL method was applied to Pinus mugo Turra needle samples collected in the subalpine zone of the High Tatra Mts (Western Carpathians), in the years 2019 and 2020. Changes in the chemical composition of samples after artificial ozonation were traced by gas chromatography/mass spectrometry (GC/MS) analysis. In addition, O3 uptake through open stomata was estimated by calculation of the modelled ozone dose (MO3D). We also conducted an inspection of visible injury (VIN) on the needle surface focused on the occurrence of O3-induced symptoms and biotic harmful agents. Regarding OxS results as well as VIN indices, P. mugo needles showed relatively low sensitivity to oxidative stress induced by O3. Therefore MO3D in a range between 14 and 16 mmol m−2 can be considered as O3 dose with minor phytotoxic effect on P. mugo growing in the mountains of central-eastern Europe.