We had previously shown that P. lividus embryos subjected to heat shock are unable to synthesize proteins of the hsp70 family at a detectable level before the hatching blastula stage. We show here that this is not due to the inability to synthesize the hsp70 mRNAs, which are detectable following heat shock also in early stages, although in much lower amounts per embryo than in later stages. These mRNAs are also translated, as judged by the facts that they are present in the polysomal pellet, and that they are translatable in a cell free system. As to the question of the amount of hsp70 RNAs per nucleus, we conclude that this is also higher in later than in earlier stages. The presence of hsp70 mRNAs is already detectable after heating at 4 centigrades above 20 and their amount increases with the increase of temperature in the range between 24 degrees C and 28 degrees C.