Data on chromosome transformation in meiotic prophase I during mammalian oogenesis are summarized. The main peculiarity of the female meiosis in mammals is an unusually long diplotene stage which may be subdivided into four periods: 1) the early diplotene (up to the beginning of follicle formation); 2) the dictyotene or "diffuse diplotene", implying primordial follicle oocytes; 3) the most pronounced lampbrush chromosome stage coinciding with the large growth period; 4) the stage of chromosome inactivation and karyosphere formation corresponding to the terminal stage of oocyte development before ovulation. These stages are associated with changes in the transcriptional chromosome activity. A correlation is revealed between the spatial chromosome arrangement in the oocyte nucleus and the transcriptional activity. Some regularities are followed in the transformation of the main nucleolar component arrangement during meiotic prophase I in mammalian oocytes. At the late pachytene and at the early diplotene, a segregation of the main nucleolar components has been observed. These components are disposed in the direction: chromatin--fibrillar center--dense fibrillar component--granulo-fibrillar component. At the dictyotene, signs of nucleolar segregation are still observed. At the lampbrush chromosome stage, when the nucleus is most highly transcriptionally active, an integration of nucleolar components occurs. At the late diplotene--prediakinesis stage, i.e. in the course of transcriptional activity lowering and karyosphere formation, the secondary segregation of the main nucleolar components occurs. These move to the nucleolar periphery to be disposed around a large fibrillar mass which is gradually displacing the rest of the nucleolar components. The fibrillar mass formation in the preovulatory oocyte nucleoli is one of the peculiarities of the diplotene and prediakinetic mammalian oocytes.