The enzyme tryptophan decarboxylase (TDC) (EC 4.1.1.28) converts tryptophan into tryptamine, and thereby channels primary metabolites into indole alkaloid biosynthesis. The production of these secondary metabolites in suspension cells of Catharanthus roseus depends on medium composition. Of the possible variables, we investigated the effect of hormones on the expression of the tdc gene in cell cultures. Omission of NAA from the growth medium resulted in accumulation of tdc mRNA. The addition of 1-naphthaleneacetic acid (NAA), indoleacetic acid (IAA) or 2,4-dichlorophenoxyacetic acid (2,4-D) rapidly reduced the enhanced tdc transcript level. Cytokinin was unable to suppress the enhanced transcript level. Hairy roots transformed by Agrobacterium rhizogenes also showed a reduction of the tdc mRNA level after NAA addition. Run-off transcription experiments showed that the down-regulation takes place at the transcriptional level within 15 minutes and independent of de novo protein synthesis. Thus one of the mechanisms which control the activity of terpenoid indole alkaloid biosynthesis in C. roseus cell cultures is the negative regulation by auxin of the gene involved in the first committed step.