Testosterone increases the size and strength of skeletal muscle. This study further characterized the molecular mechanisms of the anabolic actions of testosterone on a rat myoblast cell line (L6 cells). Testosterone did not induce hypertrophy in L6 cells lacking the androgen receptor (AR). Hypertrophy was prevented by the AR antagonist bicalutamide and the mTOR inhibitor rapamycin. Testosterone induced Erk phosphorylation by 2 h, and mTOR autophosphorylation was elevated within 20 min; phosphorylation of p70S6 kinase was increased by 2 h. Inhibitors of Erk or PI3K blocked tesotosterone-induced hypertrophy. Erk phosphorylation returned to baseline when media containing testosterone was replaced at 16 h with fresh media lacking testosterone; when bicalutamide was added to testosterone-enriched media at 16 h, Erk phosphorylation remained elevated. Autophosphorylation of the IGF-1 receptor was minimally altered by testosterone at 20 min and unaffected at later time points; PI3K/PDK1-dependent phosphorylation of Akt was not altered by testosterone. These findings indicate that testosterone stimulates hypertrophy of L6 myoblasts through a mechanism that requires its binding to the AR and involves a signaling cascade dependent upon Erk and mTOR which is likely activated by substances released into the extracellular space which are not IGF-1 or other ligands for receptor tyrosine kinases. Published by Elsevier Inc.