['Leukocyte telomere length has been shown to be associated with life span. Hypoxia-associated changes of telomere length have been detected in cell cultures, but no in vivo studies have reported the changes of telomere length under different hypoxic conditions. This study aimed to evaluate the effects of altitude on telomere length in rat leukocytes.', 'One hundred and ten male Wistar rats were randomized into 3 groups and maintained at sea-level (altitude of 10 m) (SL group, n=10), moderate altitude (2,260 m) (MA group, n=50), or simulated high altitude (5,000 m (SHA group, n=50). The last two groups were further divided into 5 subgroups and exposed to hypoxia for 1, 3, 7, 15, or 30 days (n=10). The leukocyte telomere length, hemoglobin concentration, red blood cell count, hematocrit, and plasma levels of telomerase reverse transcriptase (TERT), hypoxia-inducible factor 1α (HIF-1α), malondialdehyde (MDA) and superoxide dismutase (SOD) were measured.', 'Leukocyte telomere length was significantly longer in the MA group than in the SL or SHA groups, and the TERT expression changed in a similar manner as the leukocyte telomere length. However, HIF-1α level was significantly higher in both MA and SHA groups than the SL group. SOD level was decreased and MDA level was elevated in SHA group.'] Are telomeres elongated in rats exposed to moderate altitude?