HMG proteins were extracted with 5% PCA or 0.35 M NaCl from whole tissue, nuclei or chromatin of the liver of young (19 weeks) and old (118 weeks) male rats. They were resolved on acetic acid-urea polyacrylamide gel. The electrophoretic patterns of the major HMG proteins 1, 2, 14 and 17 of both ages are similar. The in vitro synthesis of HMG 1 and 2 decreases, but that of HMG 14 and 17 increases considerably in the liver of old rats. The synthesis of different HMG proteins is modulated differentially by spermine, butyrate, dexamethasone and 3-aminobenzamide in the liver of young and old rats. These findings suggest that HMG proteins contribute to alterations in the organization of chromatin and expression of genes during aging.