The aim of this paper was to determine the effect of temperature-humidity index (THI) on daily milk yield, depending on the dairy cow breed. The monitoring included a farm with dairy cows of the Holstein breed (H), and a farm with dairy cows of the Czech Fleckvieh (CF). Three zones were identified, which included: a no stress zone, a very mild stress zone, and a mild stress zone. The average daily milk yield was statistically signicantly (p < 0.05) influenced in the H breed during the period of the mild stress (zone 72-79). The milk yield decreased by 0.88 l per day per cow. There was found out a statistically insignificant increase of the daily milk yield in the CF breed. From these results can be concluded the CF breed is more tolerant to the heat thanks to its performance characteristics.