The composition of the H1(o) histone subfractions was examined in different rat and mouse tissues. Using reverse-phase HPLC and hydrophilic-interaction liquid chromatography we have found that the relative proportions of all four forms of H1(o) differ from tissue to tissue and from species to species. In principle, we observed an age-dependent increase in the amount of both the N-terminally acetylated (H1(o)a Asn-3 and H1(o)a Asp-3) and the deamidated forms of H1(o) (H1(o)a Asp-3 and H1(o)b Asp-3). Compared with the proportion of N-terminally acetylated H1(o) forms in liver, kidney and brain of rats and mice 20 days of age, we found an increase in these H1(o) subfractions of up to 30% in the corresponding organs of 300-day-old animals. The proportion of deamidated H1(o) forms was 1.6- to 4-fold higher in the livers and 8- to 12-fold higher in the brains of 300-day-old mice and rats, respectively, than in 20-day-old animals. The tissue-specific nature of the ratio of H1(o) subfractions suggests that the different forms of histone H1(o) have specific individual functions. The possible biological significance of age-related accumulation of N-terminal acetylated and deamidated histone H1(o) forms is discussed in the light of our results.