Tight regulation of foreign genes expressed in vivo would facilitate studies of many biologic processes and would be useful for gene transfer-based therapies. To test the ability of a tetracycline-regulated gene expression system to function in vivo, we directly injected chimeric tet repressor-VP16 transactivator expression plasmids and luciferase target genes into the hearts of adult rats. Cardiac luciferase activity increased over two orders of magnitude in response to small changes in input tetracycline-controlled transactivator DNA. Transactivation was repressed to background levels by subtherapeutic concentrations of tetracycline in a dose-dependent manner. Target gene expression could be rapidly and reversibly controlled by manipulating antibiotic administration. This system may be particularly useful for in vivo studies of gene function or gene therapies where the timing or extent of expression are critical variables.