Use of the silver-NOR method to study the nucleolar organizers in human oocytes demonstrates that topographic and quantitative variations occur during meiotic prophase. In the oogonia nucleolus the nucleolar organizers are dispersed, whereas beginning at leptotene and throughout the remaining stages of meiotic prophase they occupy a marginal position in the nucleolus. At leptotene, a modal number of seven nucleolar organizers can be observed, whereas this number falls to 2.5 at pachytene and rises to ten at diplotene, thus showing that there is intense rRNA synthesis during the latter stage of meiosis. During pachytene, one end of the bivalents containing the ribosomal cistrons is always associated with the Ag-positive zone of the nucleolus. Observation of pachytene in the electron microscope shows that the secondary constriction region of D and G bivalents is constantly associated with the fibrillar center of the nucleolus. Comparison of these two methods of investigation reveals that the silver-stained regions of the nucleolus correspond to the fibrillar centers. The latter are surrounded by a layer of electron-dense fibrils corresponding to the zone of rDNA transcription. This electron-dense layer is absent during pachytene when the nucleolus displays spontaneous segregation of its components; this absence is related to temporary arrest of rDNA transcription. The affinity of the fibrillar centers for silver-NOR staining confirms that these structures contain ribosomal cistrons. During the diplotene stage, numerous micronucleoli are formed outside the nucleolar organizers of D and G chromosomes. Most of these micronucleoli present an Ag-positive granule on one of their margins, thus indicating that they contain an actively transcribed sequence of rDNA. This observation confirms the existence of amplification of ribosomal genes in the human oocyte.