Three hundred day-old broiler Arbor Acres chicks were as­signed to three groups, 100 birds in each, fed with 0.0, 0.5 or 1,0 ppm diazepam under normal condition. At 36 days of age, each group was divided into equal two sub-groups, fifty birds in each. First: still under normal condition, second: exposed to 38°C for 4 h to 3 days. Daizepam sigificanty increased weight gain, improved feed converion and the total plasma protein of broiler chicks with or without heat stress and (T3) with heat stress. The best improve­ment was obtained with the highest level of diazepam. Diazepam redused the mortality rate, adrenalin hormone with or without heat stress and body temperature, respiration rate with heat stress. Di­azepam had slight effect on (T3), respiration rate without heat stress and minimization the increase of body temperature during heat stress. There was significant decrease on weight gain and the total plasma proteins of broiler chicks and significant increase in mortality rate. adrenalin hormone, body temperature, respiration rate, corresponding to heat stress,