Human embryos from the 2-cell to the morula stage developing in vitro after monospermic fertilization were incubated with [3H]thymidine or [3H]uridine and those possessing multinucleated blastomeres were examined by conventional transmission electron microscopy and by light- and electron-microscope autoradiography. Labelled DNA was present in all nuclei showing normal ultrastructural appearance, in pseudonuclei lacking nucleoli and their precursors and often demonstrating an unusual chromatin organization, free in the cytoplasm in structures resembling aggregates of dense chromatin and in small cytoplasmic vesicles in the close vicinity of these aggregates. The labelling with [3H]thymidine was not detected in about 50% of the cytoplasmic chromatin aggregates, suggesting that this extranuclear DNA was no longer replicated. Signs of extrusion of pseudonuclei to the extracellular space were occasionally observed. RNA synthesis could not be detected on free cytoplasmic chromatin, while [3H]uridine was incorporated into all nuclei and most pseudonuclei of multinucleated blastomeres of embryos at the 8-cell and morula stages. However, the major outburst of transcriptional activity and the ultrastructural cytoplasmic changes typical of the assumption of embryonic genome expression, occurring normally at the 8-cell stage, were observed only exceptionally.