We have studied, at the electron microscope level, the reorganizations of nucleolar ultrastructure induced by actinomycin D (AMD) in different conditions of drug treatment associated with an inhibition of rRNA synthesis. We have analyzed in parallel the localizations of ribosomal genes, of their transcripts, of various pre-rRNA intermediates, as well as of U3 RNA and fibrillarin by in situ hybridization with nucleic acid probes and immunocytological detection on thin sections of human and mouse cells. Consistent with previous observations, dense fibrillar component (DFC) and granular component (GC) appear to contain distinct pre-rRNA species at different stages of their processing. DFC appears as a major site of U3 RNA accumulation, but a very substantial fraction of nucleolar U3 RNA is also found in GC, colocalizing with partially processed pre-rRNAs. Remarkably, the major nucleolar components retain their ultrastructural appearance when extensively depleted of their pre-rRNA moiety, and ribosomal genes are always detected over fibrillar center (FC), even after extended AMD treatments which result in the characteristic segregation of nucleolar components. Moreover, while for GC the U3 RNA and pre-rRNA contents evolve in parallel following the cessation of rRNA synthesis, a dramatic uncoupling is observed for DFC. The persistent presence of U3 RNA and fibrillarin after pre-rRNA depletion suggests that DFC could represent an anchorage site for U3 snRNPs, before their entering another cycle of pre-rRNA processing reactions.