Hypertrophic responses to stretch are attenuated in slow-tonic muscles from old quail, relative to young birds. It is not known if the age-associated differences would be similar in fast-twitch muscles after stretch. This study compared the effect of 30 days of stretch overload on slow beta fibers and fast-alpha fibers in patagialis (PAT) muscles from young adult (YA) quail aged 12 weeks, old adult (OA) quail aged 52 weeks and the oldest (OO) quail, aged 90 weeks. The PAT muscle was stretched for 7, 14, 21, or 30 days by a sleeve that was attached to one wing. Birds received a subcutaneously implanted 5'-bromo-2'-deoxyuridine (BrdU) pellet that provided a constant release of 0.22mg BrdU/g body wt/day to label nuclei that had undergone DNA synthesis during stretch. Fiber cross-sectional area (CSA) was determined by planimetry from slow beta and fast-alpha fibers after identification by myosin ATPase. Harris hematoxylin was used to identify muscle nuclei. Muscle mass increased by 44.1 +/- 3.1, 32.6 +/- 3.9 and 25.7 +/- 4.3% in YA, OA and OO birds, respectively. Slow-beta fiber hypertrophy was observed at day 7 of stretch in all birds; however, YA birds had greater hypertrophy than the older birds. Fast-alpha fibers were unaffected by stretch. No significant difference was observed between PAT muscles from OA and OO birds. Total fiber number was not significantly elevated in the PAT muscle from any age group after 30 days of stretch. Activated satellite cells were not detected in stretched muscles of either young or old birds. This suggests that the increase in mean slow-beta fiber CSA was potentiated by existing myonuclei rather than by recruiting additional nuclei.