Many studies have shown that the functional architecture of eukaryotic genomes displays striking similarities in evolutionarily distant organisms. For example, late-replicating and transcriptionally inactive chromatin is associated with the nuclear periphery in organisms as different as budding yeast and man. These findings suggest that eukaryotic genomes are organized in cell nuclei according to conserved principles. In order to investigate this, we examined nuclei of different animal and plant species by comparing replicational pulse-labelling patterns and their topological relationship to markers for heterochromatin and euchromatin. The data show great similarities in the nuclear genome organization of the investigated animal and plant species, supporting the idea that eukaryotic genomes are organized according to conserved principles. There are, however, differences between animals and plants with regard to histone acetylation patterns and the nuclear distribution of late-replicating chromatin.