The methods used to achieve blastomere cell cycle synchronization in embryos used as nuclear donors during embryo reconstruction have been largely unsuccessful. The aim of this study was to determine the reliability of 6-dimethylaminopurine (6-DMAP), an inhibitor of maturation promoting factor, to halt and to synchronize blastomere division in cleavage stage bovine embryos. A second goal was to assess its reversibility and toxicity in vitro. Eight-cell stage embryos obtained at 58 h after insemination were treated with several concentrations of 6-DMAP for 12 h. Treated embryos were assessed for cleavage arrest, chromatin morphology, DNA synthesis, histone H1 and scored for blastocyst formation and for hatching rate. They were subsequently fixed and the number of nuclei counted. Complete arrest of cell division was observed at concentrations of 3 mmol 6-DMAP l−1 and above. At these concentrations, interphase nuclei in arrest were noticeably larger compared with interphase nuclei of eight-cell control embryos. Removal from 6-DMAP led to release from cleavage arrest and was followed by synchronized mitosis, histone H1 kinase deactivation and re-entry into interphase within 4–5 h. Twenty-nine per cent of interphase nuclei were synthesizing DNA at the end of the 12 h treatment as indicated by BrdU analysis. At 2 h after removal from 6-DMAP, an abrupt decrease to 9% BrdU-positive nuclei was observed followed by an increase to 39% by 6 h and a decrease to 28% at 10 h. The ability of treated embryos to reach the blastocyst stage in vitro and the number of cells per blastocyst were reduced. These results indicate that 6-DMAP can reversibly arrest and synchronize cleavage to the fifth cell cycle in eight-cell bovine embryos. Although a decrease was observed in the proportion of blastocysts obtained after treatment, it is concluded that 6-DMAP is a useful tool for synchronization studies requiring donor nuclei at metaphase before fusion to recipient oocyte.