Until recently, the transplantation of skeletal muscle across a major histocompatibility barrier has proved difficult. However, with the advent of cyclosporine (CsA), it has become possible to achieve extended survival across such histocompatibility barriers. To date, very little is known about the histochemical, biochemical, immunological or contractile properties of longterm-surviving muscle allografts. Consequently, it was the focus of this study to histochemically examine muscle allografts prolonged with CsA and determine the cross-sectional area of fast glycolytic muscle fibers. Measurements of cross-sectional area were made because they are an important correlate to the amount of tension a muscle can generate. Animals were assigned randomly to one of three groups: control (normal) (n = 5), syngeneic (n = 4), and allogeneic (n = 4). Muscle allografts were performed by transplanting the gastrocnemius of an ACI rat (RTla) hindlimb into the hindlimb of a Lewis rat (LEW;RT11). The syngeneic model consisted of an ACI-to-ACI transplant. Animals in the allograft group were given CsA (8 mg/kg/day) until the time of sacrifice. At approximately 100 days following transplantation,