The effects of heat stress at 37°C and 45°C for 8 h on the seedlings of Karacadag and Firat wheat cultivars differing in sensitivity was investigated by means of chlorophyll a fluorescence, photosynthetic pigment content and 2-D SDS polyacrylamide gel electrophoresis of the thylakoid membrane. Heat stress inhibited chlorophyll accumulation at 45°C for 8 h and caused marked alterations in the chlorophyll a fluorescence and photosynthesis in the primary leaves of the wheat cultivars at 37°C and 45°C for 8 h. Examination of the 2-D SDS polyacrylamide gel electrophoresis analysis of the thylakoid membrane proteins from the two wheat cultivars showed that while a heat treatment at 37°C and 45°C did not induce or enhance the synthesis of any protein. While the synthesis of some proteins were repressed when compared to the control temperature cultivars, the photosynthetic responses of Karacadag were less altered than Firat to the effect that; Karacadag showed lower reduction in the chlorophyll content, FV and FV/FM parameters where the F0 parameter only increased in the Firat cultivar at 45°C. Therefore, Karacadag was determined to be a heat tolerant cultivar that can be used for cultivation in warmer regions.