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This study aimed to evaluate and compare the physiological variables of three sheep genotypes: Morada Nova, Somalis, and Dorper x Somalis crossbreed regarding the environment and diet in the Brazilian semi-arid region by using 30 animals (10 from each genetic group). Five of these animals had 30% of the tifton hay replaced by hairy woodrose hay. The temperature and relative air humidity were registered, the black globe temperature and humidity index (BGHI) was calculated, and the rectal temperature (RT), respiratory rate (RR), heart rate (HR), and surface temperature (ST) were measured in the morning and afternoon. The experimental design was a completely randomized design in a 3 × 2 × 2 factorial scheme, which consisted of three genotypes, two diets, and two periods, and five replications each. BGHI presented values of 81 in the shade and 90 in the sun and a significant difference (P < 0.05) was observed for the studied variables in relation to the periods, with higher values in the afternoon. The average ST was 35.48 °C in the morning and 38.11 °C in the afternoon. Morada Nova presented the lowest RT and RR (P < 0.05) among the genetic groups, with a higher tolerance to heat. The 30% substitution of tifton hay by hairy woodrose hay did not negatively affect the thermoregulatory capacity of sheep. The studied genetic groups were adapted and tolerant to climate conditions of the Brazilian semi-arid region.