Since telomerase has been suggested to play a role in radiation response, the effect of radiation on the regulation of telomerase activity was studied on two NPC cell lines, CNE1 and 915. In order to investigate the correlation between cell survival and telomerase activity, three low doses (0.25, 0.5 and 1 Gy) and three high doses (2, 4 and 8 Gy) of gamma-radiation were used. We found that low doses of radiation induced telomerase activity (up to 250% of untreated controls) followed by a cell cycle G1 arrest, suggesting that the up-regulation of telomerase may play a role in initiating radiation induced cell cycle arrest. High doses of radiation resulted in decreased telomerase activity (down to 30% of untreated controls) and this was followed by an increased cell death. Our results indicate that the up-regulation of telomerase activity is an important response to radiation induced sub-lethal DNA damage. Our results also suggest that inhibition of telomerase activity by high doses of radiation may play a role radiation-induced cell death.