The ability to control gene expression in a temporal and spatial manner provides a new tool for the study of mammalian gene function particularly during development and oncogenesis. In this study the suitability of the tet-system for investigating embryogenesis was tested in detail. The tTACMV(M1) and rTACMV-3 (reverse Tc-controlled transactivator) transgenic mice were bred with NZL-2 bi-reporter mice containing the vector with a tTA/rTA responsive bidirectional promoter that allows simultaneous regulation of expression of two reporter genes encoding luciferase and beta-galactosidase. In both cases reporter genes were found to be expressed in a wide spectrum of tissues of double transgenic embryos and adult mice. The earliest expression was detected in tTACMV(M1)/NZL-2 embryos at embryonic day 10.5 (E10.5) and rTACMV-3/NZL-2 embryos at E13.5. Doxycycline abolished beta-gal expression in tTACMV(M1)/NZL-2 but induced it in rTACMV-3/NZL-2 embryos including late stages of embryo-genesis. The tTA and rtTA transactivators thus revealed a partially complementary mode of action during second half of embryonic development. These experiments demonstrated that both Tet regulatory systems function during embryonic development. We conclude that the Tet systems allows regulation of gene expression during embryonic development and that 'double reporter' animals like the NZL-2 mice are useful tools for the characterization of newly generated tet transactivator lines expressing tTA (or rtTA) in embryonic as well as in adult tissues.