The rate at which skeletal muscle recovers from an injury is reduced in aged as compared to young animals. To investigate age- and exposure-related changes in gene expression using Quantitative Reverse Transcription Polymerase Chain Reaction (qRT-PCR) in rats recovering from an acute exposure to repeated stretch-shortening cycles (SSCs). Fischer 344 × Brown Norway rats, aged 12 weeks (young, n = 6) and 30 months (old, n = 5), were used. The injury protocol consisted of exposing the left limb to 150 total SSC contractions, conducted at 500 degrees/s throughout a 90 degrees - 140 degrees range of motion. Animals were euthanized immediately following an isometric force test performed 10 days after the original SSC exposure. The proximal sections of the left (experimental) and right (non-exposed control) tibialis anterior muscles (LTA, RTA) were excised and flash frozen in liquid nitrogen. Expression levels of transcripts involved in muscle repair and regeneration were determined by qRT-PCR using gene specific primers. Levels of an IGF-I splice variant, referred to as mechano growth factor (MGF), were significantly greater in young animals compared to old animals (p=0.0076), and greater in the LTA (compared with the RTA (p=0.0002). Interleukin-15 (IL15) and Bcl-2 associated × protein (Bax) levels were increased in the LTA compared with the RTA (p=0.006) and (p=0.005), respectively. ?-cell leukemia/lymphoma 2 (Bcl2), was increased in the RTA compared with the LTA (p=0.02). The Bax/Bcl2 ratio was also greater in the LTA than the RTA (p = 0.0001). Aging did not affect IL-15, Bax or Bcl2 RNA levels. Conclusions: Expression levels of a number of mRNAs involved in muscle regeneration and repair are affected by SSC exposure. However, only MGF is affected by age, with transcript levels being higher in young than in old animals. Other investigators have demonstrated that MGF induction in injured muscle is reduced in aged animals. These studies also suggest that MGF plays a role in recovery during the days immediately following the injury. The data from this study indicates that MGF may also play a more long-term role in mediating muscle recovery, and in addition, may be involved in maintaining muscle composition and function.