The Merino sheep represents the most prevalent sheep breed in Poland and is one of the most genetic-stable sheep races. Therefore, the aim of the study was to determine the responses of Polish Merino sheep to three thermal conditions: thermoneutral (16.5 degrees C, group I), mild heat stress (30 degrees C, group II), and severe heat stress (50 degrees C, group III). During the experiment heart rate (HR; beats/min), respiratory rate (RR; breaths/min), rectal temperature (RT) and skin temperatures (ST) were measured daily. The ST measured at four points of the animal body was mostly statistically different (p < 0.01) between the treatment groups. It was found that the highest HR occurred in group II (30 degrees C), and that it was higher than in groups I (16.5 degrees C) and III (50 degrees C) (p < 0.01 for groups I, III). The RR was different (p < 0.01) in all experimental groups (I, II, III) with the highest increase in group II. There was also a clear and significant (p < 0.01) increase in RT during the heat stress phases. Blood analysis including morphology, biochemistry and hormones (adrenaline, noradrenaline, ACTH, and cortisol) was also performed. Significant differences in cortisol level in group II (p < 0.01) and group III (p < 0.05) were observed. There was an upward trend in adrenaline and ACTH as a result of increasing temperature. Differences in noradrenaline levels (p <0.01), with the highest level recorded in group III, were also observed. Additionally there was a strong upward trend in blood parameters, especially in hormone concentrations (cortisol, noradrenaline).