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The use of tissue expanders in reconstructive surgery is now common. However, the physiologic mechanisms by which expansion is achieved are not well understood. A recent study demonstrated that rapid expansion of skeletal muscle is accompanied by an increase in the number of sarcomeres within a muscle fiber. This is in contrast to previous animal studies whose results suggested that synthesis of sarcomere units was limited to the perinatal period. To further investigate potential increases in sarcomeres and attempt to localize the active sites of sarcomere synthesis, labeled adenosine (3H) was injected into rats during the expansion of skeletal muscle. Adenosine was taken up by the muscle fibers and incorporated in the newly formed actin as part of light chains. An autoradiographic analysis of histologic sections of the expanded muscle demonstrated a statistically significant increase in radioactivity within the expanded muscle. The distribution of the radioactivity followed a proximal-to-distal gradient, with the proximal sections exhibiting more than 50 percent greater activity than the distal aspects. These data suggest a preference for sarcomere synthesis in the proximal portion of the expanding skeletal muscle. The significance of this finding is uncertain. However, we suspect that sarcomere synthesis is tension-dependent and likely to be related to local tension applied to a portion of the muscle fibers rather than to an anatomic site of preference.