Zygote is formed by fusion of two terminally differentiated and highly specialized cells - oocyte and spermatozoon. To acquire totipotency, zygotic genome in non-human mammalian species undergoes substantial reprogramming, which mainly involves changes in epigenetic patterns, especially in DNA methylation. However, little is known about epigenetic reprogramming in human preimplantation development. We have studied DNA methylation patterns in 73 metaphase spreads of 45 human triploid embryos from 1-cell up to blastocyst stage. 5-methylcytosine-abundant DNA regions in metaphase chromosomes were identified with commercially available monoclonal antibodies (Eurogentec, Belgium). At the zygotic stage two pronuclei showed equivalent methylation pattern, while the third one was less or more methylated. This suggests different methylation status of maternal and paternal genomes in 1-cell embryos. 2-cell embryos featured hemimehtylation (asymmetric labeling of sister chromatids) of all chromosomes from triads. Hemimethylated chromosomes were typical up to blastocyst stage, but generally they decrease in number during cleavages. Hypomethylated chromosomes initially appeared at 3-cell stage and were observed in increasing number up to morula-blastocyst stages. Band-specific methylation- M-banding (Pendina et al., 2005) - was obvious in all chromosomes since morula-blastocyst stage, althou gh single metaphases demonstrated this pattern since 8-cell stage. Therefore, both paternal and maternal genomes undergo passive loss of methylation from the 2-cell to blastocyst stage. Gradual genome-wide demethylation may be caused by random combination of hypomethylated, undermethylated and methylated chromatids in daughter blastomeres during initial cleavages. Supported by CRDF and RFBR.