Oocyte-thymocyte mouse cell hybrids were produced using polyethylene glycol (PEG) and examined at the ultrastructural level. Fusion was accomplished either before or after activation of metaphase II oocytes. In both experimental variants thymocyte nuclei undergo remodelling which comprises the following sequence of events: nuclear envelope breakdown, initial chromatin condensation, and subsequent decondensation, nuclear envelope reformation and formation of nucleoli. In hybrids produced before oocyte activation but activated within a short time and cultured for several hours the thymocyte nuclei become identical to the female pronucleus. In the second variant (fusion with activated oocytes) the degree of remodeling of thymocyte nuclei is variable. Our observations demonstrate that between metaphase II, telophase of meiosis and early female pronuclear stages the mouse oocyte contains all "factors" necessary for remodelling of differentiated somatic nuclei and their development as if they were pronuclei.