By means of in situ nick-translation technique, methylation patterns of pericentric heterochromatin of chromosomes 1, 9 and 16 in extraembryonic (chorion) and embryonic cells of 5-8 week old human fetuses with normal karyotype (5), and in one specimen with trisomy for chromosome 16 were studied. Fixed metaphase chromosomes from direct chromosome preparations were digested with either endonuclease Msp I or its isoshizomer Hpa II recognizing and restricting the same sDNA sequence C decreases CGC with Hpa II, but not Msp I sensitive to methylation state of internal cytosin. According to our results, heterochromatin of extraembryonic, but not embryonic cells is hypomethylated. An obvious difference was registered in signal strength between homologous regions in iq12 of both parental chromosomes 1 in early (5-6 week old), but not in more advanced fetuses. Methylation pattern difference was detected in pericentric chromatin of triple copies of chromosome 16 in extraembryonic tissues of the 47,XY, + 16 fetus. These results are in line with a hypothesis of intraheterochromatin location of "early" genes governing initial stages of embryonic development in humans.