Two tau cDNA probes of 1.6 and 0.3 kilobases (kb) have been used to study the expression of the tau mRNAs during mouse brain development and in highly homogeneous primary cultures of neurons and astrocytes. (1) Whatever the stage, a 6-kb mRNA was detected with the two probes. In the astrocytes a 6-kb mRNA hybridized clearly only with the 1.6-kb probe. (2) During brain development the abundance of tau mRNA increases from a late fetal stage (-4 days) until birth, remains high until 6 days postnatal, and then markedly decreases to reach very low values in adulthood. Such a marked decrease in the abundance of tau mRNA parallels that of alpha-tubulin mRNA. These data suggest that: (1) depending on the stage of development and on the cell type (neurons or astrocytes) tau mRNAs of the same size encode several tau proteins differing in molecular weight: several tau proteins are expressed either during early stages of development (juvenile tau proteins of 48 kilodaltons) or in adulthood (mature tau proteins of 50-70 kilodaltons) or are specific of the astrocyte (83 kilodaltons). (2) The expression of the two major components of axonal microtubules, tubulin and tau proteins, seems to be developmentally coordinated.