الفهرس 
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Abstract
This study was performed in Nutrition and Nutritional Deficiency Diseases Dept. Fac. Of Vet. Medicine, University to: 1 Study the effect of supplementing the diets with two high levels of each of the vitamins thiamin, niacin and pyridoxine above the recommended dietary requirements on growth performance, blood metabolites and hormones and histopathological changes in the broiler chicks subjected to heat stress. 2 Two hundred and eighty eight Nubbard chicks were divided at the beginning of the 3rd week into 4 equal groups (72 each) one group act as control one and each of the other groups was subdivided into 2 equal subgroups (36 chicks each). 3 The experiment chick in subgroups were fed diets supplemented with 2 level of either thicimin (8 and 16mg/kg), niacin (108 and 216mg/kg) or pyridoxine (12 and 24mg/kg) during chicks under heat stress. 4 Body weight, body gain, consumed diets and feed conversion were recorded weekly. 5 Blood samples were collected for determination (glucosetriglyceridescholesterol total proteinalbuminglobulin, T4, T3 and cortisol), also, lymphocytes and heterophil counts and heterophil/lymphocyte ratio were calculated and studying the histopathological changes. 6 The result of experiment revealed that the dietary supplementation of broiler subject to heat stress with thiamin or niacin at concentrations of 8 or 108mg/kg, respectively improved the growth performance, biochemical parameters and minimized the pathological changes and could alleviate a part of adverse effect of heat stress.