Chromatographic comparisons of histones isolated by acid extraction of liver chromatin preparations from F344 inbred rats and their paternal heterotic F344 X H hybrid progeny reveal the presence of a number of age-and line-specific nonhistones also extractable in acid. The patterns of the core histones (H2A, H2B, H3, and H4) remain relatively constant during development, while histone H1 shows unusual chromatographic properties in the 30- and 35-day hybrid preparations. In general, the complexity of the nonhistone fraction decreases as the animal ages. Such variations may be a reflection of a prelude to altered genetic activity in organisms undergoing growth and development at different rates, and may have significant implications for transcriptional processes.