The objective of this study was to determine the effect of elevated air temperature in the first grow period on some physiological indicators of broiler chickens of different origin. Day-old Ross 308 and Hubbard Flex broiler chickens were assigned to 4 groups. Groups I (Ross 308) and II (Hubbard Flex) were kept under standard thermal conditions throughout rearing, and groups III (Ross 308) and IV (Hubbard Flex) were exposed to 10°C higher than recommended air temperature from 1 to 21 days of rearing. At 1, 21 and 42 days of the experiment, blood was collected from 10 birds in each group to determine T3 and T4, total protein, immunoglobulin complex, glucose, hemoglobin and hematocrit levels. The exposure to the thermal challenge decreased T3 and T4 levels at 21 days of rearing in both Ross 308 and Hubbard flex broilers compared to birds raised under standard thermal conditions. At 21 days of the experiment was observed a statistically significantly lower concentration of total protein in group I compared with group III and between group II and IV. There was no effect of elevated air temperature on the immunoglobulin complex concentration in the blood of birds of both genetic groups. In both genetic groups, the exposure to the thermal challenge caused a tendency to decrease the concentration of glucose. Statistically significant differences at 21 days of rearing of the hemoglobin content were observed between Ross 308 birds from groups I and III. The thermal challenge caused a statistically significant decrease in hematocrit levels in birds from both genetic groups at 21 days of the experiment. The thermal challenge upset the body’s homeostasis in both genetic groups of chickens, which possibly suggests that elevated air temperature during the first period of rearing has a negative effect on the welfare of broilers, regardless of their origin.