Several cell mediators such as growth factors and receptor ligands have been discussed to be involved in inducing cell hypertrophy reaction. Insulin-like growth factor-1 (IGF-1) is typically known to promote hypertrophy response in muscle cells and its signaling mechanisms have been elucidated. The basic fibroblast growth factor (bFGF) also appears to be a critical factor of cardiac hypertrophy as well as cardiovascular cell growth. But it is not clear how bFGF regulates such hypertrophy responses in skeletal muscles in vitro and in vivo. To investigate the possibility of hypertrophy in skeletal muscle, we constructed a replication-defective recombinant bFGF-expressing adenovirus (Ad/CMV/bFGF) and transferred it to rabbit hind limb by intramuscular injection. The rabbit skeletal muscles were successfully infected with high efficacy and the expression of bFGF was detected with immunoblotting using anti-bFGF antibody. The cells infected with Ad/CMV/bFGF revealed a significant increase in cell size over control. An average cell size of Ad/CMV/bFGF-infected tissues showed more than two times larger than that of control. To understand molecular mechanisms of skeletal muscle hypertrophy caused by bFGF, we investigated expression level of heat shock proteins and calcineurin pathway, which is thought to be factors involved in hypertrophy signaling. In tissues showing the hypertrophy induced by bFGF, IL-6 was stimulated and heat shock proteins (Hsp56 and Hsp72) were overexpressed. In addition, calcium-dependent phosphatase calcineurin was also increased and its expression level was closely correlated with that of heat shock proteins. These results strongly suggest that bFGF plays important roles in skeletal muscle hypertrophy and calcium/calcineurin-mediated signaling and heat shock proteins regulate bFGF-induced hypertrophy.