Two aL and twoβtubulin subunits are synthesized in vitro by polyadenylated mRNAs isolated from fetal and adult human cortex. The relative levels of the mRNAs encoding the different subunits change dramatically during development. In the fetus, the mRNA forβ1tubulin is present at higher levels than that of theβ2electrophoretic variant. There are relatively high levels of the mRNAs encoding both a subunits. In the adult, the levels of the mRNAs encoding both the a subunits and theβ1subunit are decreased relative to those of the mRNAs encoding theβ2subunit. These results suggest that fetal and adult cortical cells have very different requirements for the different tubulin electrophoretic variants.