The enzyme telomerase is active in germ cells and is critically involved in maintenance of telomere length in successive generations. In preimplantation mammalian embryos, telomerase activity is present from the morula stage onward and is associated with an increase in telomere length in blastocysts. Herein, we show that telomere length regulation in murine and bovine blastocysts differed between trophectodermal and inner cell mass cells in a species-specific manner. Ectopic expression of human telomerase reverse transcriptase (TERT) in bovine embryos increased telomerase activity and in turn increased telomere length. Transient expression of human TERT could be targeted to the 4-cell to morula stages and to the morula to blastocyst stages using unmodified and cytosine-methylated expression plasmids, respectively. Introduction of human TERT constructs in bovine embryos resulted in functional telomerase expression and effective telomere elongation, allowing us to study the effects on embryonic development. Ultimately, these studies may lead to a large-animal model for telomere regulation and aging.