Somatic-cell nuclei injected in Xenopus laevis oocytes remain transcriptionally active for up to 4 weeks. The rate of RNA synthesis increases as the somatic-cell nuclei enlarge. The message activity of the RNA synthesized by the injected nuclei can be demonstrated, as new proteins are made a few days after injection of HeLa-cell nuclei. Several lines of evidence demonstrate that these proteins are coded for by the HeLa-cell genome and that they result from RNA synthesized within the oocytes in the course of several days. Furthermore gene expression by the somatic-cell nuclei is selective, since only a few gene products can be detected. The oocyte cytoplasm is able to reprogramme the injected nuclei in such a way that only genes of the type normally active in oocytes are expressed. We describe experiments which indicate that genes previously inactive in the somatic cells can be switched on by the oocyte cytoplasm. These experiments have involved the use of the frog somatic-cell nuclei injected into newt oocytes. In addition, we have also found that even purified DNA is transcribed when injected into the nucleus of Xenopus oocytes. The identification of the molecules presumed to regulate chromatin transcription has been hampered by the lack of a satisfactory experimental system for assaying the biological activity of purified macromolecules. Living oocytes injected with somatic-cell nuclei (or cloned DNA) may provide such a system, since these regulatory factors could be introduced by a second injection.