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Variance components for daily milk production as well as breeding value of dairy Simmentals for daily milk production in terms of heat stress defined as different values of THI threshold values (72, 74, and 76) were estimated using 1,636,192 test-day records provided by the Croatian Agricultural Agency. Temperature-humidity index (THI) calculated from ambient temperature and relative humidity recorded in the barns on the milk recording day was used as the measure of heat stress. Bivariate estimation model accounted variances due to lactation stage, calving season, age at calving, breeding region, parity, permanent environment, interaction between hear and year, and individual animal. Obtained high genetic correlations between the daily milk production in normal and condition characterized by heat stress, as well as very high correlations between the EBVs in normal and heat stress conditions indicate animals’ high level of acclimatization to the environment on dairy cattle farms. Since microclimate measurements were carried out only once at a milk recording day, these results should be taken with caution. Generally, further research with multiple daily measurements of the microclimate parameters in the facilities is necessary to provide a fully unambiguous conclusion.