Tau is a family of microtubule associated proteins, heterogeneous in molecular weights, which are expressed specifically in neurons. Tau is encoded by a single gene, while its transcript undergoes a complex and regulated alternative splicing, giving rise to several mRNA species that migrate on Northern blots at approximately 6 and 2 kb. In this report we characterize a full size transcript of tau mRNA from rat brain and demonstrate that it contains 5203 nucleotides (not including exon 2 and 3), which correlates well with the exact size of the transcript as analyzed by Northern blot using RNA standard size markers. The full length of the 3'-untranslated region contains 3848 bp and includes two polyadenylation signals which may yield the two size transcripts in the central nervous system. The first polyadenylation signal is located in the retained intron 13/14 and the second polyadenylation signal is 19 nucleotides before the poly(A) tail. Unspliced intron 13/14 was detected in all RNA preparations tested, including RNA from different ages and different regions of rat brain, RNA from dorsal root ganglia and from undifferentiated and differentiated PC12 cells. In none of the above tissues and cells was a spliced transcript lacking intron 13/14 detected.