Temporally-regulated expression of endogenous genes is a desirable goal in stable cell line and transgenic animal systems, as well as in clinical gene therapy. Protocols for introducing genes into stable cell lines and experimental animals are often unsatisfactory due to the constitutive expression of such transgenes. To circumvent this problem we have demonstrated specific and temporally regulable expression of a target gene in vivo effected by a chimeric regulator in response to an orally-administered, non-toxic chemical. This regulatory system utilizes a chimeric regulator GLVP, consisting of a mutated human progesterone receptor ligand binding domain (PRLBD-delta) fused to the yeast GAL4 DNA binding domain (DBD) and the HSV VP16 transcriptional activation domain and whose activity is solely regulable by non-physiological doses of RU486 but not by progesterone or other endogenous progestins. Replacing the activation domain of the chimeric regulator with a transcriptional repression domain results in inducible repression of target gene expression in vitro. Our regulatory system functions in transient and stable transfections as well as in transgenic animals, and will have a wide variety of potential applications.