Telomerase synthesizes telomeric DNA repeats onto chromosome ends de novo. The mouse telomerase RNA component was cloned and contained only 65 percent sequence identity with the human telomerase RNA. Alteration of the template region in vivo generated altered telomerase products. The shorter template regions of the mouse and other rodent telomerase RNAs could account for the shorter distribution of products (processivity) generated by the mouse enzyme relative to the human telomerase. Amounts of telomerase RNA increased in immortal cells derived from primary mouse fibroblasts. RNA was detected in all newborn mouse tissues tested but was decreased during postnatal development.