The objective of this study was to evaluate the thermoregulatory and physiological responses of Najdi rams raised under hot summer conditions prevailing in Kingdom Saudi Arabia. The core temperature (T_core) showed circadian rhythm characterized by biphasic achrophases, which were associated with the feeding times during both seasons. Average daily T_core was significantly (P<0.05) higher under hot summer conditions. However, the amplitudes of the rhythmic oscillation during winter and summer seasons were 0.85 and 0.56^o C, respectively. Rectal (T_re) and skin temperatures (T_sk) were significantly (P<0.05) higher under hot summer conditions and exhibited similar patterns during both seasons concurrent with the pattern of temperature humidity index (THI), reaching the maximum values late in the afternoon and the minimum values early in the morning. Respiratory rate (RR) and heart rate (HR) showed the same pattern of the thermal parameters and were significantly (P<0.05) higher under hot summer conditions. Serum concentrations of total protein, globulin, glucose, sodium and chloride were significantly (P<0.05) increased while those of albumin and calcium were decreased under hot summer conditions. The results obtained from this study indicate that hot summer conditions of Saudi Arabia is thermally stressful to Najdi rams. Therefore, Najdi sheep production under such conditions would require environmental and/or nutritional modification to alleviate the impact of heat stress.

Key words: Albumin, Glucose, Heat load, Sheep, Thermoregulation