الفهرس 
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Abstract
ascorbic acid or vitamin c is widely used in poultry production for reducing heat stress. the present studies aimed to study the pathological effects of heat stress on broiler chicks and to detect the most proper level of ascorbic acid that minimize these detrimental effects so , one hundred and twenty broiler chicks (2weeks old) were used in this work and the broilers were divided into two categories in two rooms each room contained 4 groups, each group contained randomly selected 15 birds. the broiler groups (1, 3, 5, 7) in the first room were subjected to heat stress (3739 oc for 7 hrs daily for 45day ), while broiler groups (2, 4, 6, 8) in the other room maintained under normal environmental conditions. group 3 and 4, group5 and 6 , group 7 and 8 were supplemented with aa at levels 330, 550 and 1000 mg / kgm diet respectively. the criteria used to evaluate efficiency of ascorbic acid supplementation included pathological findings and biochemical studies were obtained at each scarification at two weeks interval. the result revealed that, histopathological lesions were found in different organs of broilers exposed to heat stress but more severe in group (1) and less in groups (3, 5 and 7), including: congested hepatic blood vessels and hemorrhage in the liver, round cells infiltration in portal areas and focally replacing liver cells and hyDROPic degeneration in hepatocytes. congestion in coronary arteries, hemorrhage, edema in myocardium and hyaline degeneration in cardiac muscles. congestion and hemorrhage in kidneys, focal replacement with round cells and hyDROPic degeneration in renal epithelium. congestion, hemorrhage and edema in lungs. congestion, hemorrhage and depletion of lymphocytes from lymphoid organs. congestion and round cells infiltration in lamina propria of proventriculus and mucosa of intestine. congestion in brain and meninges. congestion, hemorrhage and hyaline degeneration in thigh muscles. biochemical studies supported histopathological results, in that ascorbic acid supplementation alleviating effect of heat stress . we are concluded that , supplementation of heat stressed bird with ascorbic acid minimize the detrimental effects of heat stress particularly at level of 1000mg/ kgm diet.