Alternative splicing of human tyrosine hydroxylase (hTH) transcripts appears to occur mainly in the N-terminal domain, giving rise to at least eight different isoforms. We recently reported the existence of hTH transcript variants resulting from splicing of exons 8 and 9, within a region previously thought to be constant. The mRNA distribution of these novel hTH isoforms in neuroblastic tumours and in foetal adrenal glands was analysed by conventional and real-time RT-PCR. The presence of the target protein was determined by Western blotting, immunoprecipitation and protein analysis. Transcripts lacking exons 8 and 9 were widely distributed in the tissues analysed. Characterisation of full-length mRNA revealed that splicing of exons 8 and 9 was always associated with splicing of exons 2 (hTH-Delta2,8,9) or 1b and 2 (hTH-Delta1b,2,8,9). In addition, one variant detected on Western blots in several tumours fits the predicted size (58 kDa) of the isoforms lacking exons 8 and 9. In conclusion, the two novel isoforms reported here (hTH-Delta2,8,9 and hTH-Delta1b,2,8,9) represent the first full-length isoforms with alternative splicing in the hTH C-terminal domain. These results demonstrate for the first time the existence of hTH isoforms Delta2,8,9 and Delta1b,2,8,9. Their general distribution in neuroblastoma and adrenal glands and translation into protein suggest a significant functional role for these novel hTH isoforms, which merit further study.