Transcription of the ribosomal RNA (rRNA) genes occurs in the nucleolus resulting in ribosome synthesis. The rRNA gene activation and the associated nucleolus formation may be used as a marker for the activation of the embryonic genome in mammalian embryos and, thus, serve to evaluate the developmental potential of embryos originating from different embryo technological procedures. In bovine in-vitro-produced embryos, functional ribosome-synthesizing nucleoli become structurally recognizable towards the end of the fourth postfertilization cell cycle. A range of important nucleolar proteins (e.g. fibrillarin, nucleophosmin, nucleolin, topoisomerase I, upstream binding factor and RNA polymerase I) become localized to the nucleolar anlage over several cell cycles and this localization is completed towards the end of the fourth cell cycle. In nuclear transfer embryos originating from day 5-6 invitro-produced bovine embryos, fully developed nucleoli are apparently detected one cell cycle later, and a substantial proportion of the nuclear transfer embryos display aberrations in protein localization in one or more blastomeres. This information may help to explain the abnormalities observed in a proportion of fetuses and offspring derived from nuclear transfer embryos.