Stress or heat shock proteins (hsp) are synthesized by most cells in response to adverse environmental conditions. In mammalian cells, the major proteins synthesized in response to stress have relative molecular weights (Mr) in the range of 68 to 74 kilodaltons (kD) and are encoded by a small multi-gene family collectively referred to as hsp 70 genes. In unfertilized mouse eggs, no members of the hsp 70 family appear to be synthesized under normal or stressful (heat shock) conditions. At the two-cell stage, two proteins with Mr = 74 kD and Mr = 70 kD are expressed as a consequence of developmentally activated transcription of these hsp 70 genes. No stress-induced synthesis in response to heat shock is observed at this stage. At the eight-cell stage, constitutive synthesis of the 70-kD protein continues, but, as in the two-cell embryo, no heat shock induced synthesis of a novel heat shock protein is observed. By the blastocyst stage, however, an inducible protein with Mr = 68 kD is synthesized in response to heat shock in addition to constitutive synthesis of the 70-kD protein. The constitutively synthesized cognate proteins are coded for by a set of mRNAs about 2,000 nucleotides in length. The induced hsp 68 proteins are coded for by mRNAs of larger size (about 2,600 nucleotides). Only the smaller mRNA class is detectable on Northern blots of RNA extracted from control or heat shock cleavage stage embryos and control blastocysts. As predicted from protein synthetic studies, both classes are resolved in RNA preparations derived from heat-shocked blastocysts.