The Cre/loxPsite-specific recombination system derived from bacteriophage P1 provides a convenient tool for directed modifications of genomes in various organisms. To exploit Cre-mediated manipulation of mouse genomic sequences at the zygote stage, we have developed a transgenic mouse line carrying the CAG-cretransgene in which thecregene is under control of the cytomegalovirus immediate early enhancer-chicken β-actin hybrid (CAG) promoter. The activity of the Cre recombinase at early stages of development was examined by crossing the CAG-cretransgenic mice to another transgenic mouse line carrying a reporter gene construct, CAG-CAT-Z, which directs expression of theE. coli lacZgene upon Cre-mediated excision of theloxP-flanked chloramphenicol acetyltransferase (CAT) gene located between the CAG promoter and thelacZgene. PCR-based analysis of F1 progeny from CAG-cremales × CAG-CAT-Z females showed that transmission of the CAG-cretransgene was accompanied by the complete deletion of the CAT gene of the CAG-CAT-Z transgene in all tissues, and that this deletion was never observed in the progeny without transmission of the CAG-cregene. On the other hand, analysis of F1 mice from CAG-CAT-Z males × CAG-crefemales showed that the CAG-CAT-Z transgene had undergone complete deletion of the CAT gene in all tissues irrespective of the cotransmission of the CAG-cregene. This Cre-mediated recombination in F1 mice occurred before the two-cell stage of embryonic development, as shown by X-gal staining. The results suggest that the CAG-cretransgene is expressed in developing oocytes of CAG-cretransgenic mice, and Cre mRNA and/or protein are retained in mature oocytes irrespective of the transmission of the CAG-cretransgene, resulting in efficient Cre-mediated recombination of paternally derived target genes upon fertilization. The CAG-cretransgenic mouse should serve as a useful tool to introduce prescribed genetic modifications into the mouse embryo at the zygote stage.