Telomeres, the G-rich sequences found at the ends of eukaryotic chromosomes, ensure chromosome stability and prevent sequence loss from chromosome ends during DNA replication. During macronuclear development in Tetrahymena, the chromosomes fragment into pieces ranging from 20 kb to 1,500 kb. Tetrahymena telomerase, a ribonucleoprotein, adds telomeric (TTGGGG)n repeats onto telomeres and onto the newly generated macronuclear DNA ends. We have investigated whether telomerase RNA levels increase during macronuclear development, since such an increase might be expected during chromosomal fragmentation. The steady-state level of the telomerase RNA component was used to estimate the abundance of telomerase present in mating and nonmating Tetrahymena. Northern blot analysis revealed that in vegetatively growing Tetrahymena, there were 18,000-40,000 copies of telomerase RNA per cell. In mating cultures, the levels of RNA increased 2- to 5-fold at 9-15 h, and 1.5- to 3.5-fold in starved nonmating cultures. This increase in telomerase RNA paralleled telomerase activity, which also increased slightly in mating and starved nonmating cells.