Cryoelectron microscopy of vitrified sections has become a powerful tool for investigating the fine structural features of cellular compartments. In the present study, this approach has been applied in order to explore the ultrastructural morphology of the interphase nucleus in different mammalian cultured cells. Rat hepatoma, Chinese hamster ovary and Potorus kidney cells were cryofixed by high-pressure freezing and the cryosections were examined at low temperature by transmission electron microscopy. Our results show that while the contrast of nuclear structural domains is remarkably homogeneous in hydrated sections, some of them can be recognised due to their characteristic texture. Thus, condensed chromatin appears finely granular and the perichromatin region contains rather abundant fibro-granular elements suggesting the presence of dispersed chromatin fibres and of perichromatin fibrils and granules. The interchromatin space looks homogeneous and interchromatin granules have not been identified under these preparative conditions. In the nucleolus, the most striking feature is the granular component, while the other parts of the nucleolar body, which appear less contrasted, are difficult to resolve. The nuclear envelope is easily recognisable with its regular perinuclear space and nuclear pore complexes. Our observations are discussed in the context of results obtained by other, more conventional electron microscopic methods.