The intranucleolar chromatin has been detected by feeding HeLa cells with bromodeoxyuridine and then using an immunocytochemical technique for its detection at the ultrastructural level. Labelling was mainly observed on the dense fibrillar component (DFC), which surrounds the fibrillar centres (FC). Labelling could also be seen on the network within the granular component (GC) of the nucleoli. Ring shaped nucleoli with large FCs showed labelling, in addition, on the FCs. Nucleoli from spread preparations of cells treated with hypotonic salt solution for different periods revealed a nucleolonemal network, consisting of units comparable to rDNA transcriptional units in length and associated with tufts of fibrils and granules. Thread-like axes could be detected running through the units and in between two units. Chromatin could also be identified within these units by the immunolabelling technique. The observations strongly indicate that these units are identical with the rDNA transcriptional units. A dynamic nature of the nucleolar chromatin and nucleolar organization has been discussed.