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The influence of hyperthermia (33 ºC, 2 days) on the ultrastructure of palisade cells of mesophyll of the first rosette leaves of arabidopsis Columbia 0 ecotype (Col-0, phases 1.02–1.04) was studied. Samples of 12-day-old seedlings were selected in 2 variants: control and 2 days 33 ºC. Seedlings of the control variant were grown in a growth chamber with a photoperiod of 15/9 hours. (day/night), illumination 5.5 klx, 75 % humidity and temperature 22 ºC. In the experimental variant containers with 9-day-old seedlings were transferred for 2 days to a growth chamber with a preset light 5.5 klx and temperature 33 ºC, with a photoperiod of 15/9 hours. The conducted ultrastructural analysis allowed to reveal the spectrum of rearrangements of palisade cells after two-day action of high (33 ºC) temperature. It was shown that the high temperature negatively affected size of mesophyll palisade cells, the cross-sectional area of which was 12 % smaller than in the control. Chloroplasts show an increase in granality: in the control granas contained 6–10 thylakoids, often combining into larger granas, up to 20 or more thylakoids in the intersection zone, while after two-day hyperthermia the granas contained 20 or more thylakoids, often forming giant granas of 60 and more thylakoids, the average cross-sectional area of starch granules decreased by almost half: 0.99 μm2 compared to 1.92 μm2 in the control, the diameter of plastoglobuli increased 3–4 times: to 100–200 nm compared to 30–50 nm in the control. In mitochondria, there was a decrease in the partial volume of the cristae, enlightenment of the matrix, the cross-section of mitochondria increased at least twice: 1 μm2 compared to 0.44 μm2 in the control. The mean cross-sectional area of peroxisomes also increased at least twice, to 1.36 μm2 compared with 0.77 μm2 in the control.