In the present study, immunofluorescence confocal laser scanning microscopy, autoradiography following (3)H-uridine incubation, and transmission electron microscopy were used to evaluate the nucleolar protein localization, transcriptional activity, and nucleolar ultrastructure during genomic reprogramming in bovine embryos reconstructed by nuclear transfer from in vitro-produced bovine morulae to activated cytoplasts. During the first cell cycle (one-cell embryos), no autoradiographic labelling was detected. Ultrastructurally, whorls consisting of densely packed fibrillar material were observed instead of nucleoli. During the second, third, and fourth cell cycle (two-, four-, and tentative eight-cell embryos), autoradiographically unlabelled nuclei contained vacuolated bodies consisting of densely packed fibrillar material. Also, during the fourth cell cycle, the first nucleoplasmic autoradiographic labelling was observed, but still without formation of fibrillo-granular nucleoli. During the fifth cell cycle (tentative 16-cell embryos), the nuclei displayed autoradiographic labelling over both nucleoplasm and presumptive nucleoli, and the formation of fibrillo-granular nucleoli was observed. In a certain proportion of blastomeres, however, abnormal patterns of nucleolar formation and apoptosis were noted. During the first two cell cycles, labelling of RNA polymerase I, fibrillarin, upstream binding factor (UBF), nucleolin (C23), and nucleophosmin (B23) was localized to nuclear entities. During the third cell cycle, labelling of topoisomerase I was observed in addition. During the fourth and fifth cell cycles, a substantial portion of the embryos presented blastomeres that lacked labelling of several of these nucleolar proteins. In conclusion, the nuclear transfer procedure was associated with remodelling of the nucleoli to an inactive form, followed by reformation of fibrillo-granular nucleoli during the fifth cell cycle. Moreover, a certain proportion of blastomeres failed to form functional nucleoli with respect to both ultrastructural organization and protein allocation.