Purpose: The effects of chronic exercise training on the expression of heat shock protein (HSP) in skeletal muscle of senescent animals are unknown. Therefore, the purpose of this study was to investigate the effects of chronic exercise training on skeletal muscle HSP expression in both young and old rats. Methods: Young adult (3 months) and old (23 months) female Fisher 344 rats were assigned to either a sedentary control or an endurance exercise trained group (N = 6 per group). Exercised animals ran (60 min.d(-1), 5 d.wk(-1)) on a treadmill at similar to 77% (V) over dot O-2peak for 10 wk. After completion of the training program, the soleus (SOL), plantaris (PL), and the red (RG) and white portions (WG) of the gastrocnemius muscles were excised, and citrate synthase (CS) activity and the relative levels of HSP72 were determined. Results: Training resulted in increases (P < 0.05) in (V) over dot O-2peak in both young (67.6 +/- 3.1 vs 86.9 +/- 1.6 mL.kg(-1).min(-1)) and old animals (54.5 +/- 1.8 vs 68.2 +/- 2.2 mL.kg(-1).min(-1)). Training increased CS activity and the relative levels of HSP72 (P < 9.05) in all four skeletal muscles in both young and old animals. Specifically: compared with age-matched sedentary controls, exercise training resulted in increased (P < 0.05) levels of HSP72 in skeletal muscles of both young (SOL +22%, PL +94%, RG +44%, WG +243%) and old animals (SOL +15%, PL +73%, RG +38%, WG +150%). Conclusions: These findings reveal that the exercise-induced accumulation of HSP72 in skeletal muscle differs between fast and slow muscles. Further, these data indicate that the exercise-induced accumulation of HSP72 in highly oxidative skeletal muscles (SOL and RG) is similar between young and old animals. In contrast, aging is associated with a blunted expression of HSP72 in fast skeletal muscles (PL and WG) in response to chronic exercise.