Protein phosphorylation catalyzed by the cAMP-dependent protein kinase is implicated in transcriptional activation of the embryonic genome in the two-cell mouse embryo, while heat shock protein (hsp70) has been identified as one of the first products of zygotic gene activation. Using reverse transcription-polymerase chain reaction we have analyzed relative changes in the amount of hsp70 mRNA during oocyte maturation and early embryogenesis. We report that the amount of hsp70 mRNA decreases after germinal vesicle breakdown, while inhibiting germinal vesicle breakdown inhibits this maturation-associated decrease. The amount of hsp70 mRNA increases between the one- and two-cell stages. This increase is inhibited by either alpha-amanitin or the cAMP-dependent protein kinase inhibitor H-8; the same concentration of H-7, which is a more potent inhibitor of protein kinase C, has little inhibitory effect on this increase in the relative amount of hsp70 mRNA. Last, addition of cycloheximide to one-cell embryos late in G2 inhibits neither cleavage to the two-cell stage nor the increase in the relative amount of hsp70 mRNA. These results strengthen the previous proposal that protein phosphorylation is involved in zygotic gene activation in the two-cell mouse embryo.