الفهرس 
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Abstract
One hundred and three crossbred ewes of mixed ages belonging to Desert Institute, at Maryout Egypt were used to investigate the effect of season of mating and lambing on ewes productivity and certain physiologicl responses. The influence of heat stress on some physiological mechanisms, both in pregnant and lacating ewes and also in their growing lambs was wconsidered therefore. 3 climatic controlled ( 35 c and 55 % RG ) experiments were carried aut at the UMC, USA. The results from this study are discussed in more detail and this results leads to the following conclusions: (1) the physiological state of animals (Pregnancy and loctation) is important considering responses to environmental alterations. (2) loctation appears to be a stresful stimulus of greater physiological influences in comparison with pregnancy, either under thermoneutral heat stress conditions. (3) the inhibition of lactogensis in response to heat induced hyperthermia could be primarily due to depressed metabolic activities of the mammary secretary tissue cells. Depressed feed intake and depressed thyroid gland activities could be contributing factors.