Follicular atresia is characterized by apoptosis of granulosa cells, and telomerase plays an important role in the apoptotic process. To study the relationship between follicular atresia and telomerase activity, we investigated changes in telomerase activity and localization in experimentally induced atretic follicles of immature rats. Immature female Sprague-Dawley rats received 15 IU equine CG (eCG) by subcutaneous injection. Rats were killed under ether anesthesia at 2, 3, 4, or 5 days after eCG injection. Telomerase activity in granulosa cells was measured by telomeric repeat amplification protocol (TRAP) assay, and telomerase localization in the ovary was examined by in situ TRAP assay. Telomerase activity was detected at high levels in granulosa cells on day 2 after eCG injection regardless of follicle size, and levels were significantly decreased in large follicles, with atretic changes, on days 4 and 5. No such decrease was observed in granulosa cells of small follicles. In the next experiment, rats received subcutaneous injections of estradiol (1, 10, or 50 microg/rat) to prevent follicular atresia or sesame oil as a control from day 2 to day 4 after eCG injection and were killed under ether anesthesia on day 5 after eCG injection. The changes observed in the large follicles on days 4 and 5 in oil treated rats were not observed with estradiol treatment. These findings suggest that the telomerase in granulosa cells is likely to play an important role for healthy follicle life and that loss of its activity may be associated with follicular atresia.