During the last years and due to its economic importance sheep farming expands at flat land areas in Greece exhibiting less favourable climatic conditions especially during summer. It is therefore justifiable to assess the potential summer heat-stress of sheep growing at areas of different landscape. Potential heat-stress of sheep during summer was studied at three Greek husbandry areas of different landscape, namely Larissa - flat land, Ioannina - semi-mountainous and Trikala Korinthias - mountainous. Indices used were the night hours during which ambient temperature was below 21ºC, the Temperature Humidity Index (THI), the time percentage (%) within predefined heat-stress categories and the THI-hrs index. Overall, the area of Larissa exhibited the worst heat-stress conditions. Average ambient summer temperatures were above 21ºC during the whole 24 h period, whereas at Ioannina and Trikala Korinthias average temperatures were below 21ºC for almost half of the day including night. Daily average THI values were 27.2±0.2 for Larissa, 21.8±0.2 for Ioannina and 21.3±0.2 for Trikala Korinthias. During the hottest and the coolest summer days the average daily THI values at the area of Larissa were higher than those at Ioannina, which were also higher than at Trikala Korinthias. At Larissa the time percentage (%) within the extreme severe heat-stress category (IV) was significantly (P<0.05) higher, namely 58.3%, compared to Ioannina (34.3%) and Trikala Korinthias (9.2%). Average (2010-2014) THI-hrs under heat-stress were 11491 for Larissa, 5722 for Ioannina (49.8% of Larissa) and 1868 for Trikala Korinthias (16.3% of Larissa). Expansion of sheep husbandry at flat land areas and design criteria (e.g. breed used, feeding strategy, housing density, floor type, etc.) within sheep facilities should be implemented very cautiously.