The study analysed the relationship between temperature-humidity index THI and dairy cow production in the climatic conditions of Central Bohemia. Milk yield was significantly affected by the day in milk, cow breed (Holstein, Czech Simmental) and lactation order. The effect of THI was statistically significant, with a steady decline in milk yield from THI=70. Genetic selection for heat tolerance depends on the expression of a suitable phenotype, which can be described by physiological parameters (respiratory rate, body temperature), milk yield decline or associated biomarkers defined on the basis of selected milk components.