Human genomic DNA fragments containing catechol O-methyltransferase (COMT) sequences were isolated and the exon-intron structure analysed by sequencing, PCR and comparing to the human COMT cDNA sequences. The gene contains six exons, of which exons 1 and 2 are noncoding. MB-ATG and S-ATG codons, responsible for the initiation of translation of the membrane-bound (MB) and soluble (S) forms of the enzyme, are located in exon 3. Two distinct COMT-specific transcripts, 1.3 kb and 1.5 kb, were detected in various human tissues and cell lines. Different quantities of the shorter COMT-specific mRNA in the tissues studied suggest a tissue-specific regulation of the COMT gene at transcriptional level. Mapping of the 5' ends of the COMT mRNAs showed that transcription initiates at multiple sites in two separate DNA regions, which are preceded by functional promoter sequences. The proximal promoter (P1), located between the two translation initiation codons and extending approximately 200 bp upstream of the MB-ATG initiation codon, apparently gives rise to the 1.3-kb S-COMT mRNA (S-mRNA). The distal promoter (P2) is located in a DNA fragment in front of and partly overlapping the transcription-start region of the 1.5-kb transcript, suggesting that it controls the expression of this MB-mRNA. Similarities between the rat and human COMT gene promoters are analyzed.