Molecular mechanisms of development and disease can be studied in transgenic animals. Controlling the spatial and temporal expression patterns of transgenes, however, is a prerequisite for the elucidation of gene function in the whole organism. Previously we reportted that mice carrying a tetR/VP16 hybrid gene (tTA), under the control of the hunman cytomegalovirus immediate early 1 (HCMV‐IE1) gene promoter, can be used to temporally activate the expression of transgenes under the control of a promoter containing tetop sequences. We now show that the MMTV‐LTR can be used to target expression of tTA to the epthelial cells of secretory organs and skin in transgenic mice. Notably, nearly uniform expression of a tetop‐lacZ transgene was found in seminal vesicle, salivary gland, and Leydig cells of mice carrying also the MMTV‐tTA transgene. More heterogeneous patterns of gene expression were observed in mammary epithelial cells and nasal cells of the epidermis. Different MMTV‐tTA lines had conparable tissue expression patterns. Transcriptional activation mediated by tTA was up to several hundred fold and it was abrogated after the administration of tetracycline. The MMTV‐tTA mice established in this work will be useful for experiments examining the roles of biological factors at defined developmental stages in the epithelial cells of salivary gland, seminal vesicle, mammary gland, and skin and the Leydig cells of testes in addition, in combination with the CRE/lox recombnation system, these mice will be useful to achieve gene deletions at defined time points in these organs. © 1995 Wiley‐Liss, Inc.