The effects of DNA methylation inhibitor 5-azacytidine (5-aza-C) and histone acetylation inhibitor trichostatine A (TSA) on the structure of pericentric heterochromatin in cultured mouse cells (L929) has been studied. After 48 h of 5-aza-C treatment, about 85% of the cells demonstrate transformation of chromocenters from ovoid to elongated structures. Hypotonic treatment of these cells reveals tandemly arranged DAPI-positive globules, well distinguishable by light microscopy. The same globular units can be revealed in hypotonic-treated control cells. 48 h of TSA treatment causes dramatic decrease in HP 1alpha content in the cells. Chromocenters in 25% of treated cells became highly decondenced and can not be reliably detected by light and electron microscopy. 85% of cells demonstrate globular chromocenters with low HP 1alpha content. Hypotonic treatment causes transformation of compact chromocenters into ring-like structures, which can be either single or clustered. Rings are formed by uniform fiber, in which no globular subunits are detected. The data obtained are discussed concerning several mechanisms of heterochromatin structure maintenance and the roles of epigenetic marks in them.