It has been shown by in vitro translation of polyadenylated messenger RNAs (poly(A)+ mRNAs) that the mRNAs encoding both alpha and beta tubulin isotypes are present at much higher relative levels in the developing rat brain than they are in the adult, suggesting that the requirements for tubulin subunits vary with cell type and/or with the developmental stages of a particular cell type. The postnatally developing rat cerebellum, with its readily identifiable cell populations that perform the gamut of developmental tasks, is a suitable model for analyzing specific cellular mRNA distributions during development. In this report, by in situ hybridization techniques it is shown that, by comparison to total cellular poly(A)+ mRNA levels, there is relatively more of the total beta tubulin mRNAs in mitotically active external granule layer cells than in those in the internal granule layer. These results show that migration and differentiation of these granule cells is accompanied by a decrease in their beta tubulin mRNA levels relative to the levels in granule cells of the external granule cell layer. Furthermore, the relative levels of beta tubulin mRNA both in the prenatally formed Purkinje cells and the postnatally formed stellate cells are two to fourfold less than in the granule cells of the internal granule cell layer.