The horse family, Equidae, is being used as a model system to explore the evolution of highly reiterated DNA sequences (buoyant density satellite DNAs) within mammalian genomes. Of particular interest is the relationship between the DNA sequences found in heterochromatin of species with high diploid number, e.g. Equus przewalskii, 2N equals 66, and the heterochromatic DNA of species with lower diploid chromosome number and altered patterns of heterochromatin distribution, e.g. Equus zebra hartmannae, 2N equals 32. Buoyant density satellite DNAs are to be purified and localized by in situ hybridization. Restriction enzyme analysis and filter hybridization will provide information concerning nucleotide sequence homologies.