Effects of high-temperature stress (HTS) and PEG-induced water stress (WS), applied separately or in combination, on the functional activity and ultrastructure of the photosynthetic apparatus (PSA) of maize (Zea mays L.) and sunflower (Helianthus annuus L.) plants were investigated. In maize plant tissues WS provoked the decrease in RWC by 10.9 %, HTS by 7.0 %, and after simultaneous application of the both treatments the decrease was 32.7 % in comparison with control plants. Similar but more expressed changes were observed in sunflower plants. Sunflower was more sensitive to these stresses. Net photosynthetic rate decreased significantly after all treatments, more in sunflower. In mesophyll chloroplasts after separately applied WS and HTS the number of grana and thylakoids was reduced and electron-transparent spaces appeared. At combined stress (WS+HTS) granal and stromal thylakoids were considerably affected and chloroplast envelope in many of them was partially disrupted. and I. Dekov, T. Tsonev, I. Yordanov.