Sixteen Wistar rats were randomly divided into four groups. One group was put at an ambient temperature of 23℃—25℃ as control. The other groups were exposed in an artificial climatic chamber to DB 40℃, WB 32℃, BG 46℃ and Rh 60％. These rats were decapitated after the rectal temperature rose to 41℃, 42℃ and 43℃ respectively for one hour. Isolation of total RNA from control and heat-shocked rat liver by phenol/chloroform/iscamylalcohol method and fractionation of Poly(A)⁺mRNA by Oligo (dT)-cellulose chromatography were performed. The difference of Poly(A)⁺mRNA between control and heat-shocked rat liver was shown by urea agarose gel electrophoresis. In vitro translation of Poly(A)⁺mRNA in wheat germ system, the synthesis of four major heat shock polypeptides with approximate molecular weights of 71000, 90000, 98000 and 110000 Dalton was observed. The relative amount of major heat shock proteins inereased as the rectal temperature of the heat-exposed rats rose. These results indicated that HSP was atso induced when rats, as a whole, were exposed to a simulated hot enviroment.