Conditional transgene expression in the heart is a useful approach to explore the physiological basis of the cardiac phenotype. The present study describes the development of a binary transgenic system in which transgene expression in the mouse heart can be turned on/off by administration/withdrawal of an exogenous compound. We generated a transgenic line (alphaMHC-Glp 65) harboring a mifepristone (RU486)-controlled chimeric transcription factor (Glp 65) under the regulatory control of the cardiac-specific alpha-myosin heavy chain (alphaMHC) promoter. In the presence of RU486, Glp 65 expressed in the heart is able to bind to a target gene promoter containing four copies of the 17-mer GAL4 binding site, resulting in ligand-inducible transactivation of the target gene. We tested this system by crossing the transgenic mice, alphaMHC-Glp 65, with a transgenic line harboring human growth hormone (hGH) target gene. We observed that expression of hGH could be induced in adults as well as in the embryonic hearts of bigenic mice by RU486. The basal hGH expression was very low and the inducible level in the heart was estimated over 800-fold higher versus the basal level after 4 days of administration of RU486 at 500 microg/kg body weight per day at 2-5 months of age. The level of the transgene returned to the basal level within 7 days after withdrawal of RU486. This system can be used to control cardiac-specific expression of transgene in a time- and dose-dependent manner.