Xenopus laevis erythrocyte nuclei have been used as a source of inactive ribosomal RNA genes to study the regulation of gene activity during early embryogenesis. Addition of an oocyte extract to the erythrocyte nuclear transcription assay brings about transcription by RNA polymerase I. This is not seen If an egg extract is used even though the amounts of RNA polymerase I are the same in both preparations. The oocyte-treated nuclei synthesize ribosomal RNA as defined by RNA-DNA hybridizations and sucrose gradient separation techniques. The active component of the oocyte extract has been semipurified to a single peak by Sephadex G100 column chromatography and DEAE cellulose salt elution. Its presence and absence during oogenesis and early embryogenesis parallels the transcription of the ribosomal genes in vivo at these stages. Within the oocyte itself, the active component is located exclusively in the germinal vesicle. The structure of the rlbo6omal genes in treated nuclei has been studied by DNase I digestion. The ribosomal genes in erythrocyte nuclei are Insensitive to DNase I, but treatment by an oocyte extract or egg extract plus the semipurified active component causes the entire gene repeat to become DNase I sensitive. An egg extract alone has no effect. The semipurified active component by itself causes only the 5' end of the transcribed sequence to become DNase 1 sensitive. The component therefore appears to act on the non-transcribed ribosomal genes of the erythrocyte nucleus to alter their chromatin conformation to a more DNase I sensitive one which permits RNA polymerase 1 transcription. The DNase I sensitivity of histone H4, globln, tRNA and 5Sooc RNA genes was also studied. The extracts had no effect on the DNase I sensitivity of any of these genes, but those genes transcribed by RNA polymerase 111 are DNase I sensitive whatever their transcriptional state.