mRNAs for Spec 1 and Spec 2 of Strongylocentrotus purpuratus and LpS1 of Lytechinus pictus accumulate only in the aboral ectoderm of developing embryos. In vitro nuclear transcription assays were done to study the transcriptional regulation of these cell type-specific genes. Spec 1, Spec 2c, and Spec 2d genes all appeared to be transcriptionally activated at the late cleavage-early blastula stage of S. purpuratus. Differences in the relative transcription rates during development appeared to play a major role in determining the relative levels of the various Spec mRNAs. The L. pictus LpS1 gene was transcriptionally activated at a similar developmental time as the corresponding S. purpuratus genes. Nuclei from gastrula or pluteus ectodermal and endodermal/mesodermal cell fractions were used to demonstrate that Spec 1 and LpS1 genes were transcriptionally active in ectoderm nuclei but not in endoderm/mesoderm nuclei, suggesting that in vivo the Spec 1 and LpS1 genes are spatially controlled at the transcriptional level. Estimations of the absolute rate constants for Spec 1 transcription were made at the late cleavage, mesenchyme blastula, and midgastrula stages. Calculations using these rate constants and the known levels of Spec 1 mRNA suggested that Spec 1 mRNA stability gradually increased throughout development.