Nuclear transplantation and cell fusion techniques have proved valuable for embryological studies in several non-mammalian animal species1. More recently these procedures have been used successfully in small laboratory mammals, notably the mouse, to investigate the ability of nuclei and cytoplasm from various sources to produce viable embryos when combined2–6. The use of a similar approach to study the developmental biology of large domestic animals presents a number of technical and practical difficulties, and so far there has been no report of attempts to perform nuclear transplantation in sheep embryos. Here I describe such a procedure and its use to investigate the development of embryos in which whole blastomeres from 8- and 16-cell embryos were combined with enucleated or nucleated halves of unfertilized eggs. The procedure involves bisection of single-cell eggs in a medium containing cytochalasin; fusion of egg halves with single blastomeres, induced using Sendai virus or an electrofusion apparatus; and embedding in agar, followed by culture of the reconstituted embryos in the ligated oviducts of ewes in dioestrus. I show that fully viable embryos may be obtained by this procedure.