ABSTRACT The nucleolus, the most prominent feature of the inter-phase nucleus, is involved in ribosome biogenesis (Warner, 1990). During evolution, the nucleolus first appeared at the time when nuclear envelope compartmentation of the cells was taking place. I would like in this review to provide information showing that the nucleolus is a particular nuclear territory in respect of the compartmentation of nuclear functions. In eukaryotic cells, the nucleolus is the site at which ribosomal gene transcription takes place and the machinery necessary for the production of the ribosomal subunits is assembled (Hadjiolov, 1985). At this site, the 18 S, 28 S and 5.8 S RNAs are synthesized and assembled with proteins such as ribosomal protein SI (Hügle et al. 1985a) and 5 S RNA.