Résumé : The nucleoli of the premeiotic, pachytene and young diplotene (A, B, C, up to 100 μm diameter) oocytes are heavily labelled after 7 hr of [3H]uridine incorporation, which is in good agreement with the hypothesis that nucleolar multiplication occurs throughout all these early stages of oogenesis. The total labelling of the nucleolar, nuclear and cytoplasmic volumes has been calculated from the uridine high resolution autoradiograms and the estimated amounts are discussed. The chromatin of several premeiotic oocytes and the cap of all the pachytene oocytes are strongly labelled after 7 hr of [3H]thymidine incorporation. The authors did not find any thymidine incorporation in the young A, B or C diplotene oocytes.