The birds are sensitive to stressor factors. Heat stress is important among stressor agents. By increase of temperature more that the conformity of the bird with temperature is imbalanced and it leads to reduction of bird performance. The purpose of this study was to investigate the effects of heat stress on Newcastle disease and performance of broiler breeders. In this research two sections of a big broiler hen farm were studied. All nutritional and management factors like vaccination, nutrition, density and water resource and environment were the same and only the temperature was different during production. In farm 1 the temperature was high and uncontrolled, but in farm 2 the temperature was controlled by temperature control system. In farm 1 the clinical and autopsy symptoms of Newcastle disease and falling of production until 18% and 4.5% death were observed as a result of increase of temperature after 20 days. In farm 2, the outcomes of heat stress was not seen, because of controlling temperature in houses by ventilation and cooling system. It should be pointed that the average of Newcastle vaccines titer was 7 and its CV average was 18 before production in both farm. There was a significant difference among blood and biochemical factors like lymphocyte, and heterophil in both farm (p<0/05).
