Skeletal muscle satellite cells (SC) play an important role in muscle adaptation. In untrained individuals, SC content and activation status has been observed to increase in response to a single bout of exercise. Muscle fiber characteristics change considerably when resistance exercise is performed chronically, but whether training status affects the activity of SC in response to a single bout of exercise remains unknown. We examined the changes in SC content and activation status following a single bout of resistance exercise, prior to and following a 16wk progressive resistance training (RT) program in fourteen young (25±3yr) men. Before and after RT, percutaneous biopsies from the vastus lateralis muscle were taken prior to a single bout of resistance exercise and after 24 and 72h of post-exercise recovery. Muscle fiber size, capillarization, and SC response were determined by immunohistochemistry. Following RT, there was a greater activation of SC after 24h in response to a single bout of resistance exercise (Pre:1.4±0.3,24h:3.1±0.3 Pax7+/MyoD+ cells/100 fibers) as compared to before RT (Pre:1.4±0.3,24h:2.2±0.3 Pax7+/MyoD+ cells/100 fibers, p<0.05); no difference was observed 72h post-exercise. Following 16wk of RT, MyoD mRNA expression increased from basal to 24h after the single bout of exercise (p<0.05); this change was not observed prior to training. Individual capillary-to-fiber ratio (C/Fi) increased in both type I (1.8±0.3 to 2.0±0.3 C/Fi, p<0.05) and type II (1.7±0.3 to 2.2±0.3 C/Fi, p<0.05) fibers in response to RT. Following RT, enhanced activation of SC in response to resistance exercise is accompanied by increases in muscle fiber capillarization.