Stretch overload-induced activation of satellite cells in the anterior latissimus dorsi (ALD) muscle was examined in full-grown adult (12 wk old) and aged (90 wk old) Japanese quail. 5'-Bromo-2'-deoxyuridine (BrdU) constant-release pellets (0.22 mg BrdU.g body wt-1.day-1) were implanted subcutaneously before weighting the left wing of each bird. Nuclei that incorporated BrdU were localized by immunohistochemistry after 1 or 2 wk of stretch overload. Total fiber number was quantified by counting all fibers in a histological cross section from the midbelly of the ALD. Aging reduced increases in ALD mass and fiber number during 2 wk of stretch overload. Fiber proliferation in the ALD of aged birds also demonstrated an altered time course. The percentage of BrdU-positive nuclei associated with muscle fibers and the percentage of fibers associated with BrdU-positive nuclei did not differ between age groups. The altered time course of new fiber formation in the ALD of aged birds during 2 wk of stretch overload does not appear to be related to the capability to activate satellite cells.