Bleomycin-induced lung injury causes increased fibroblast numbers in the lung and pulmonary fibrosis. Studies of fibroblasts isolated from such injured lungs have revealed evidence of increased intrinsic proliferative capacity, but the mechanism is unknown. Telomerase catalyzes the addition of telomeric DNA repeats onto chromosomal ends, which is associated with increased cellular life span or immortality. To examine whether telomerase might play a role in regulating fibroblast proliferative capacity in pulmonary fibrosis, lung fibroblasts were isolated from rats treated with endotracheal injections of phosphate-buffered saline or bleomycin. At selected time points, the rats were killed and lung fibroblasts isolated. The isolated cells and lung tissue were then used in experiments for measurement of telomerase activity. The results show undetectable telomerase activity in fibroblasts isolated from control uninjured lungs, or in the control lung tissue extracts. Similar results were obtained in cells and lung tissue from Days 1, 3, and 28 bleomycin-injured lungs. However, significant telomerase activity was detected in fibroblasts and tissue extracts isolated from Days 7, 14, and 21 bleomycin-treated rat lungs, with maximal activity observed in the Day 14 samples. Analysis of the isolated cells for telomerase messenger RNA or reverse transcriptase expression, combined with α –smooth-muscle actin expression by immunohistochemistry, revealed that telomerase expression localized primarily to nonmyofibroblasts. These findings suggest that in addition to elevated growth factor expression, the injured lung fibroblast population may contain cells with increased life span, which could contribute to the observed overall increase in lung fibroblast numbers.