The chimeric transcriptional activator tTA, a fusion between the Tn10 encoded Tet repressor and the activation domain of the Herpes simplex virion protein VP16, was stably expressed in transgenic tobacco plants. It stimulates transcription of the beta-glucuronidase (gus) gene from an artificial promoter consisting of 7 tet operators and a TATA-box. Tetracycline, which interferes with binding of tTA to operator DNA, reduces gus expression over several orders of magnitude. This stringency of regulation suggests that the system can be used to construct transgenic plants encoding a potentially lethal gene product. Furthermore, the specific and fast inactivation of tTA allows study of the stability of RNAs and proteins.