High temperature stress is a major environmental stress factor for all photosynthetic organisms. Some lichen species could have the ability of tolerance against global warming. In the present study, we investigated the effects of high temperature on GABA metabolism in two different lichen species Evernia prunastri and Usnea sp.. Evernia and Usnea sp. were collected from unpolluted locations in Bilecik, TURKEY. Evernia and Usnea sp. were kept at 45 degrees C for 0, 24 and 48 h. We analyzed GABA content, glutamate dehydrogenase (GDH) and glutamate decarboxylase (GAD) activities and also chlorophyll and MDA contents in the thalli of the lichens. The chlorophyll degradation and lipid peroxidation data indicated that E. prunastri thalli showed tolerance to high temperature while Usnea sp. thalli were found to be sensitive under these conditions. GABA content was enhanced by high temperature stress in E. prutastri thalli, while GAD and GDH activities were decreased. According to our results, we can suggest that GABA accumulation in lichen thalli could occur via different metabolic pathways.