All modern mammals contain a distinctive, highly repeated (> or = 50,000 members) family of long interspersed repeated DNA called the L1 (LINE 1) family. While the modern L1 families were derived from a common ancestor that predated the mammalian radiation approximately 80 million years ago, most of the members of these families were generated within the last 5 million years. However, recently we demonstrated that modern murine (Old World rats and mice) genomes share an older long interspersed repeated DNA family that we called Lx. Here we report our analysis of the DNA sequence of Lx family members and the relationship of this family to the modern L1 families in mouse and rat. The extent of DNA sequence divergence between Lx members indicates that the Lx amplification occurred about 12 million years ago, around the time of the murine radiation. Parsimony analysis revealed that Lx elements were ancestral to both the modern rat and mouse L1 families. However, we found that few if any of the evolutionary intermediates between the Lx and the modern L1 families were extensively amplified. Because the modern L1 families have evolved under selective pressure, the evolutionary intermediates must have been capable of replication. Therefore, replication-competent L1 elements can reside in genomes without undergoing extensive amplification. We discuss the bearing of our findings on the evolution of L1 DNA elements and the mammalian genome.