Fibroblasts from bleomycin-injured lungs express telomerase activity transiently during the period of active fibrosis, but the signal(s) responsible for its induction is (are) unknown. The objective of this study was to identify potential mediators capable of regulating telomerase activity induction in rat lung fibroblasts during pulmonary fibrosis. Lung fibroblasts from control (NRF) and bleomycin-treated (BRF) rats were isolated and treated in vitro with either basic fibroblast growth factor (bFGF) or interleukin-4 (IL-4). At selected time points after treatment, the cells were analyzed for telomerase activity, as well as telomerase reverse transcriptase (TERT) mRNA and protein by reverse transcriptase/polymerase chain reaction and Western blot, respectively. The results showed that bFGF could induce telomerase activity in NRF and stimulate further the induced activity in BRF. The bFGF effect was accompanied by increased TERT protein expression and a rapid but transient increase in TERT mRNA. In contrast, IL-4 inhibited the induced telomerase activity in BRF, which was accompanied by increased α -smooth muscle actin expression, an indicator of myofibroblast differentiation. These findings suggest that telomerase expression could be induced in rat lung fibroblasts by bFGF, but suppressed by IL-4, which promoted myofibroblast differentiation. The latter is consistent with the preferential expression of telomerase activity in fibroblasts relative to myofibroblasts.