During the placentation process, the expression of various glycoproteins plays an important role in embryonal development. Alterations in DNA methylation caused by 5-azacytidine (5azaC) can disturb normal glycoprotein expression as well as the proliferative ability of trophoblast cells. In order to assess this, a single dose of 5azaC was injected intraperitoneally into pregnant rats during days 1-19 of gestation. Animals were euthanised on day 20 and placental weight, as well as glycoprotein composition, was analysed together with immunohistological assessment of the degree of proliferation of the trophoblast cells. The placental weight was found to be significantly smaller in animals treated by 5azaC during days 4 to 14 of gestation (p<0.01, Student's t-test). The treatment on days 4, 5, and 6 resulted in a lack of labyrinth with the strong proliferative activity of the cells in the basal layer. Expression of glycoproteins with molecular mass smaller than 60 kDa was reduced with treatment on day 6. The 5azaC administered from days 7 to 10 completely disturbed the placental structure and the proliferation of trophoblast cells was poor. During these days GP70 exhibited stronger expression in treated animals, contrary to GP40, which was stronger in controls. A natural border between the labyrinth and the basal layer was established on days 11 and 12. The basal layer was dominant with a lower proliferation of trophoblast cells compared with the controls. With the establishment of the labyrinth on day 13, the expression of GP40 was restored. Proliferation of the trophoblast cells from days 13 to 15 was higher compared with the controls. The changes in placental mass and the proliferative ability of trophoblast cells in rat placenta exposed to 5azaC represent more proof of the importance of epigenetics in the regulation of placental development.