We have determined that the stockpile of maternal histone mRNAs for both H2B and H3 in eggs of the sea urchin Strongylocentrotus purpuratus corresponds to 0.8–1 million molecules per egg. This value is a somewhat smaller quantity than had previously been estimated. On the other hand, the increase brought about by embryonic mRNA synthesis is 10- to 12-fold, rather than 4-fold. Further analysis of the time course of accumulation of these mRNAs suggests that histone gene expression in early development involves a complex interaction between activation of stockpiled maternal messengers, fluctuating rates of histone gene transcription, and alterations of histone mRNA stability. The maternal pool of H1 mRNA is considerably smaller (80,000 molecules). During cleavage, it partially catches up and peaks at about half the value for H2B and H3. We also examined previtellogenic oocytes for the presence of histone mRNAs by the same methods and were unable to detect any. Thus the stockpile of maternal histone mRNAs must accumulate after the previtellogenic stage of oogenesis.