DNA-dependent RNA polymerase I from E. coli has been used as probe for determining the capacity of purified chromatin isolated from inbred and hybrid rats during the course of postweaning development to serve as template for RNA synthesis in vitro. An analysis of variance reveals both strain- and age-specific differences in the incorporation of [3H]UTP into RNA. The ability of inbred chromatin to support synthesis exceeds that of the hybrid at every age examined except 50 days, with values in the inbred line approaching those in the hybrid line with increasing age. These data suggest that template characteristics may vary substantially with regard to strain and age and may contribute to differences in heterotic development.