Aromatic L-amino acid decarboxylase (AADC, EC 4.1.1.28) catalyzes the decarboxylation of L-dopa to dopamine in catecholamine cells and 5-hydroxytryptophan to serotonin in serotonin-producing neurons. This enzyme is also expressed in relatively large quantities in nonneuronal tissues such as liver and kidney, where its function is unknown. Neuronal and nonneuronal tissues express AADC mRNAs with distinct 5' untranslated regions. To understand how this is accomplished at the genomic level, we have isolated rat genomic DNA encoding AADC. The organization of the AADC gene suggests that there are two separate promoters specific for the transcription of neuronal and nonneuronal forms of the AADC message. A small exon containing 68 bases of the neuronal-specific 5' end is located approximately 9.5 kilobases upstream of the translation start site, which is contained in the third exon. Approximately 7 kilobases upstream from the neuron-specific promoter is another small exon containing 71 bases of the 5' end of the nonneuronal AADC message. These data suggest that transcription initiating at distinct promoters, followed by alternative splicing, is responsible for the expression of the neuronal and nonneuronal forms of the AADC message.