The experiment was carried out on 10 sheep and 10 cross-bred sheep (˝Finnish landrace x Rahmani)and 10 cross-bred goats (˝Damascus x Baladi). The age of sheep and goats ranged from 2-4.5 years old. Animals were confirmed in semi-open pens day and night throughout the period of the experiment. These pens provided enough shade and ventilation in summer and protection from rain in winter. The experiment was carried out during the hottest month of summer season, (August, 1999). Ambient temperature (AT), black body temperature (BBT) and relative humidity (RH) were recorded simultaneously while measuring the physiological responses. The following physiological parameters were taken before and after 3 hrs of exposure to solar radiation:

Rectal temperature (RToC), skin temperature (SToC), ear temperature (EToC), wool surface temperature (WSToC) and hair surface temperature (HSToC, respiration rate (RR), respiratory minute volume (GV), tidal volume (TV), VO2, VCO2, RQ, heat production, hemoglobin concentration (Hb), hematocrit value (HT%), plasma total protein (TP), plasma albumin (AL), plasma globulin (GL), albumin/globulin ratio (A/G), creatinine concentration, serum alanine aminotransferase (ALT), aspertate aminotrasferase (AST), plasma triiodothyronine (T3), plasma thyroxine (T4), plasma aldosterone (Ald), plasma Na+ and K+ concentrations.

Results indicate that: Under shade, sheep and goats had almost similar body temperatures (RT, ST and ET). However, sheep had significantly higher metabolic rate (HP) than goats which was compensated by significantly higher respiratory evaporation (RR, GV and TV) in sheep than in goats. The significantly higher T3 and T4 in goats reveal that the higher HP in sheep than goats was due to compensate the significantly higher HP in sheep than in goats.

Exposure to solar radiation increased significantly body temperatures (RT, ST and ET) indicating that the significant increase in respiratory evaporation (RR, GV and TV) was inadequate. The increase in heat production and body temperature after exposure indicate that both sheep and goats were under heat stress (ambient temperature exceeded the upper critical temperature of sheep and goats). The main thermoregulatory mechanisms were the significant increase in respiratory evaporation and a significant decrease in thyroid hormones to minimize the increase in metabolic rate. It is worthnoting that exposure to solar radiation had no adverse effect on liver (ALT and AST) or kidney (creatinine) functions. However, exposure caused a significant reduction in plasma Na+ and K+ concentrations which was accompanied with a reduction in plasma aldosterone concentration especially in sheep.

The significantly more increase in ST, RR, HP and less reduction in Ald indicate that sheep is more tolerant to heat stress due to direct exposure to solar radiation than goats may be due to the insulative properties of wool coat. Also, exposure to solar radiation caused a significant reduction in plasma K+ concentration of goats may be due to high sweating rate and high plasma Ald concentration.