Nuclear structures were studied in the growing chicken oocytes by means of light cytochemistry of nucleic acids, autoradiography and electron microscopy. The morphofunctional rearrangements of chromosomes in the oocytes at the vegetative phase are similar with those described for sexually mature hens. The nucleolus is found only in those oocytes which begin to grow prior to the sexual maturation of females. The oocyte nucleus is characterized by the highest synthetic activity at the "lampbrush" stage. The presence of a large amount of transosomes and macrobodies in the ooplasm, as well as autoradiographic analysis of 3H-uridine incorporation and labelled RNA localization within the oocyte-follicle system suggest that in the chick, as well as in adult birds, the follicular epithelium cells, together with the oocyte nucleus, take part in furnishing the ooplasm with ribonucleic acids.