The gene encoding the mouse egg primary receptor for sperm, a zona pellucida glycoprotein called ZP3, is expressed exclusively in growing oocytes within ovaries of sexually immature and mature female mice. We have constructed a transgene in which 6.5 kilobases of ZP3 gene 5'-flanking sequence is fused to the coding region of the firefly luciferase gene, and we have generated four independent lines of transgenic mice. In these animals, the transgene is expressed exclusively in ovaries. Furthermore, within ovaries, expression is confined to growing oocytes, and luciferase activity can be detected by assaying individual, isolated oocytes. The pattern of firefly luciferase expression during oocyte growth is similar to that observed in previous studies of ZP3 expression during oogenesis in mice. Observations reported here strongly suggest that cis-acting elements present in the ZP3 gene 5'-flanking region regulate oocyte-specific and, therefore, sex-specific expression of the sperm receptor gene during mouse development. They also suggest that such elements can be used to direct expression of cloned genes specifically to oocytes of transgenic mice and to evaluate the effects of such expression on various aspects of early mammalian development.