Electrophoretic profiles of acid-extractable proteins from Holtzman rat liver chromatin display four minor and five major histone bands through certain stages of postweaned development but are qualitatively different from the chromatin protein profiles previously reported during postweaned development for the Fisher 344 rat strain and the F344 X Holtzman heterotic progeny [Tallman, G., et al. (1979). Biochem, Genet. 17:185]. The protein profiles from the heterotic progeny do not reflect and are not combinations of the profiles from the parental strains. Levels of in vitro transcription with Escherichia coli RNA polymerase of total chromatin and of acid-extracted chromatin from Holtzman rat liver tissue fluctuate in an age-specific manner during postweaned development and are higher than previously published levels determined for the Fisher and F344 X H strains during the same developmental period [Tallman, G., et al. (1978). J. Hered. 69:282]. The degrees of stimulation in the transcription assays resulting from the acid treatment vary with the age of the animal but are similar for the maternal Holtzman and hybrid strains. These studies suggest that regulation of heterotic growth may involve dominant, or maternal genetic influences.