Tm aim of the present work was, on one hand, to identify, by 3H-thymidine labelling, the different types of ovarian cells in which DNA synthesis takes place in very young Xenopus laevis females and, on the other hand, to localize the DNA in these early stage oocytes, by applying the method of “staining” with radioactive actinomycin we have described elsewhere [3, 41. The animals analized, numbering 6, were aged from 2 to 3 months, that is just at the beginning of metamorphosis or 3, 4, 5, 6 and 7 weeks later. Methyl 3H-thymidine (The Radiochemical Centre, Amersham, England) of specific activity 500 mC/mM was administered intraperitoneally, using 30 ,uC per female. The animals were sacrificed 24 h after injection of the radioisotope. Ovaries were fixed by freeze-substitution and were sectioned 8 p thick. Some preparations were submitted to prior digestion with DNAase (Worthington, 0.1 mg/ml +MgCl, M/300, at 37°C during 1 h). The “staining” of unlabelled sections of ovaries with 3H-actinomycin (Schwarz, Bioresearch, specific radioactivity 2.6 C/m&‘) was carried out in the dark, during 1 h, in a solution having a radioactivity of 5 &/ml; this was followed by washing with cold actinomycin (10 pm/ml) for 30 min in the dark, then by washing with running water for 12 h. The classical autoradiographical procedure [8] was applied to both types of preparations, both being developed after 15 days’ exposure. The autoradiographs show that: (1) Thymidine incorporation into nuclear DNA takes place in the ovaries of the female which underwent metamorphosis 4 to 7 weeks before. The labelling is found, essentially in the prophase figures of premeiotic cells and in the nuclei of the smallest oocytes (these having a diameter of 30 to 50 ,u) (Fig. 1). The germinal vesicles of larger oocytes remain unlabelled. Preparations treated with DNAase show no trace of radioactivity. (2) The binding of actinomycin occurs in the follicular cells nuclei. Actinomycin is also strongly fixed in the nuclei of the very young oocytes, having a cell diameter of 30 to 150 EC, in contrast with what is found in the germinal vesicles of mature oocytes [4]. Radioactivity diminishes regularly during the increase of volume of the germinal vesicle. The activity is condensed around the nucleoli and it probably originates from the chromatin associated with the nucleolus (Fig. 2). In the nuclear sap, it seems closely related to structures which probably correspond to the lampbrush chromosomes at the beginning of their expansion (Fig. 3). At this stage, no significant labelling by actinomycin can be found in the cytoplasm.