Summer climates causes stress in dairy cows leading to depress production. Heat stress increases maintenance energy requirements, lowers dry matter intake, especially forage intake, making it difficult to meet energy needs. The most noted negative effects of heat stress are decreased milk yield. The objective of this paper was to evaluate the effect of high temperatures on dairy cow milk production in southern Slovakia. We tested the hypotheses that the milk yield is impacted by the breed (Red Holstein, Slovakian Pied cattle and Holstein) and by the cooling (fan or fogger+fan), and the season. Production data included 57,927 test-day records belonging to 20 herds. Four herds were Red Holstein cattle (9,365 records), 8 herds had Slovakian Pied cattle (22,936 records), and cows of Holstein breed were in 8 herds (25,626 records). All herds were situated in lowlands, from 130 m to 182 m above sea level. Herds were distributed according to cooling of cows during high temperatures. First group of cows (10 herds) was evaporative cooled before milking by water spray cooling equipment installed in the holding pen twice daily. Second group of cows (10 herds) was cooled using only forced ventilation (automatically controlled fans without misting in housing and feeding areas). In the months June to August were recorded 84 summer and 50 tropical days. Eighty four days were with the temperature-humidity index above 72.0 and 50 days above 78.0. The highest performance for the year was found in Holstein cows (9637.8 ± 405.0 kg; P<0.01). Evaporative cooled cows produced more milk than non-cooled (9234.4 ± 387.8 versus 8288.4 ± 392.7 kg; P<0.001). The average monthly milk yield was non-significantly higher in evaporative cooled cows for the whole period with high temperatures (from May to September). The highest milk production per month was found in Holstein cows and the lowest in Slovakian Pied cattle (P<0.05). In the season comparison, the highest production was recorded in the period January-April (P<0.001). It is apparent performance; well-being and health of animals are influenced by biometeorological factors. The most important climatological factors are heat stress during the hot season and the wind-chill factor during the cold season of the year (Fuquay, 1981). Heat stress occurs when a dairy cow's heat load is greater than her capacity to dissipate the heat. Critical temperatures will vary depending on several factors including degree of acclimatization, rate of production, pregnancy status, body weight, exposed body surface, hair coat, air velocity, relative humidity, sweating, radiant heat and temperature (Yousef, 1987; Berman, 2005). Life stage, conditioning, and nutritional and health status also influence the level of vulnerability to environmental stressors (Hahn, 1999). To maintain body temperature within the normal range, all cows, especially high producing cows, must exchange this heat with the environment (Debrecéni et al., 2001; Brouk et al., 2003). Genetic diversity within a population can also influence the level of response and the degree of adaptability, so what is stressful for some may not be stressful for others (Johnson, 1987). Cattle are adaptable to a wide range of climatic conditions, but can be severely challenged by sudden heat waves (Nienaber et al., 2003). Obsah