In cultured eucaryotic cells, heat treatments specifically induced the rapid synthesis of the so-called heat-shock polypeptides. To ascertain the physiological importance of this phenomenon for highly differentiated organisms, we attempted to determine whether the heat-shock response occurs in a living endothermic organism at extreme temperatures, and if so, whether the response is organ specific. We developed a procedure to label proteins efficiently in 5- to 18-day-old chicken embryos. Heat-shock polypeptides of identical sizes of 85,000, 70,000, and 25,000 daltons were synthesized predominantly in chicken embryo fibroblasts and in many different organs of 18-day-old embryos at 42.5 to 44 degrees C.