During the last century, the global temperature on the surface of the Earth grew for 0.180C, and climate projectional model summed up in the IPCC reports indicates that during the XXI century the warming (process) will continue to grow reaching the values of 1.1 to 6.40C (IPCC, 2007). In the parallel line with the global warming the number of scientific researches which demonstrate the negative effect of high temperatures over the production, morbidity and mortality of the animals is increasing. The weak performances, imuneosupression and the high rate of mortality which are results from exposures to high temperatures are a direct threat for the poultry production (Bottje &Harrison;, 1985; Young, 1990; Mujahid et al., 2005, 2009b).
Although the hens are homeothermal animals which maintain permanent body temperature, in comparison to the other domestic animals, they are too sensitive to thermal changes in the environment. The thermo neutrality zone of the comfort ambience temperature for the hens is 18-240C and within its frames the excess of body temperature is lost by means of conduction(Wolfenson et al., 2001), convention (Mitchell, 1985; Tzschentke et al., 1996; Lott et al., 1998) and radiation of the amount of created heat is in balance with the amount of the heat given away. In cases of increased ambience temperature and their approximation to the body temperature of the hen (410C), the given away of the heat through the above mentioned ways is impossible. The only way of giving away heat is through gasping as a respiratory evaporative mechanism (Richards et al., 1968, 1970, 1976; Seymour, 1972; Marder & Arad, 1989). Going out of the comfort zone is followed by a series of physiological changes which have adaptation as an aim i.e. keeping of the body temperature(Simon, 2003).
The reduced production of eggs in conditions of thermal stress owes to reduced circulation of blood through the womb (Wolfhenson et al.