Hepatocyte nuclei from young (3--5 week), mature (8--12 week), and aged (over 32 weeks) rats were isolated and characterized by flow cytometry. Nuclei were bulk separated into diploid (2C), tetraploid (4C), and octoploid (8C) enriched fractions on sucrose gradients. Total, 0.35 M NaCl soluble, and residual proteins were prepared from all nuclear stages and examined by one- and two-dimensional gel electrophoresis. Within limits of sensitivity of these techniques, the following general features emerged. (a) A majority of proteins visualized were common to and present in similar relative quantities in nuclei from all age and ploidy groups. (b) A relatively higher proportion of nonhistone proteins (NHP) were saline-soluble in 2C nuclei from young rats than at any subsequent stage of development. (c) Several age-related and to a lesser extent ploidy-related fluctuations in pattern among the NHP were evident. These reflected primarily differences in solubility rather than major quantitative changes among individual proteins. (d) Exceptions to the foregoing included a group of high molecular weight components (greater than 100 000), a major and a minor component between 45 000 and 50 000, and a heterogeneous group of proteins in 2C nuclei from very young animals. There were no obvious differences among the histones, although these proteins were not examined in detail. The complex pattern of changes observed are discussed in terms of known aspects of hepatocyte differentiation and are related to possible changes in nucleoplasmic, nuclear matrix and Hn-RNP associated proteins.