The immunohistochemical expression of insulin-like growth factor I and the transferrin receptor was investigated in regenerating rat skeletal muscle. Muscle injury was induced in the extensor digitorum longus muscle in one hind limb by tourniquet ischemia. The regenerating muscles were investigated 3 to 7 days after the injury. Expression of IGF-I and the transferrin receptor was demonstrated in cryostat sections by a doublestaining method. It was found that both markers were transiently expressed by the regenerating muscle cells. Expression of IGF-I immunoreactivity preceded that of the transferrin receptor and the staining for IGF-I disappeared 1 to 2 days before the staining for the transferrin receptor. The data are compatible with a regulatory role of IGF-I on the transferrin receptor.