A library of cDNA clones was constructed representing polysomal polyadenylated RNA of mesenchyme blastulae of Strongylocentrotus purpuratus. Using this library, we determined whether or not individual RNA species are associated with animalization of embryos by zinc ions. Clones corresponding to the most actively synthesized RNAs during the period just prior to the mesenchyme blastula stage were selected by screening colonies with in vivo-labeled RNA. The most abundant of these were chosen for further study. Individual RNA abundance was measured as percent of mass of total polyadenylated RNA by hybridizing cDNA exhaustively with cloned DNA on filters. The RNAs in the selected, cloned sequences were present in abundances of 0.01 to 1% of the mass of polyadenylated RNA. Changes in abundance of individual RNA species occurred during normal development and departures from these developmental changes occurred in the zinc-animalized embryos. Two RNA species, which normally increase 10-fold in abundance, are drastically repressed and at least one RNA species increases in abundance dramatically in the animalized embryos. These departures from the normal program of presumptive gene expression may furnish insights into changes in the normal processes of development.