In the normal development of Xenopus laevis, genes for oocyte-type and somatic-type 5S RNAs are both expressed in late blastulae. Estimates of rates of synthesis indicate that the oocyte-type genes (5Sooc) undergo at least a 100-fold reduction in transcriptional activity between the end of oogenesis and the late blastula stage, and at least a further 20-fold reduction during gastrulation. When neurula nuclei, with inactive 5Sooc genes, were transplanted to enucleated eggs, the resulting nuclear-transplant embryos showed a reactivation of 5Sooc genes in blastulae and a subsequent inactivation after this stage. This effect is not explicable by a differential stability of the two types of 5S RNA. We conclude that egg or early embryo cytoplasm must contain components which can continuously regulate 5S gene expression, and that the mechanism by which 5Sooc genes are developmentally inactivated does not persist through mitosis in early embryos. These results have been obtained by a new procedure in which oocyte- and somatic-type 5S RNAs are separated in a 4 M urea-15% acrylamide gel.