Glycogen synthase kinase 3 beta (GSK-3beta) is a multifaceted serine-threonine kinase that is of interest both because of its role in the canonical Wnt signaling pathway, which is involved in mammalian brain regionalization, and its role in phosphorylating the microtubule-associated protein Tau. Because of the potential association of GSK-3beta with human developmental and neurodegenerative conditions, we determined its exon/intron boundaries by a combination of sequencing, polymerase chain reaction (PCR) and database mining. Study of GSK-3beta expression using reverse transcription-PCR, Western blotting and Northern blotting showed alternative splicing in nervous and non-nervous system tissues. Both at the protein and mRNA level we were able to identify two isoforms, one full length form containing exon 10 and one without exon 10. At the mRNA level we identified an additional exon that is sometimes seen between exons 8 and 9. Furthermore, rather than the reported 2-3 kb mRNA predominant in non-neural tissues, we identified the major brain isoforms of GSK-3beta as two high molecular weight RNAs (8.4 and 7.7 kb).