Fluoro-uracil (FUc) and fluoro-uridine (FU) inhibit the maturation of pre-rRNA to 18S and 28S rRNA in Xenopus oocytes and cell lines. As a consequence of the incorporation of the nucleotide analogue, a heterogeneous population of RNA molecules heavier than 2.6 million D is conserved. A certain proportion of these molecules has a size of about 4 million D (45S). It is shown by hybridization experiments, that part of the large transcripts induced by the drugs are ribosomal precursors which contain sequences complementary to most of the rDNA repeating unit, including the so-called non-transcribed spacer.