Insulin-like growth factor-I (IGF-I) is a potent growth factor that regulates and promotes growth in many types of cells and tissues, including skeletal muscle. The igf1 gene encodes mature IGF-I and a carboxy-terminal extension called the E-peptide. In rodents, alternative splicing and post-translational processing produce two E-peptides (EA and EB). The mature IGF-I produced by both isoforms is identical, while EA and EB share less than 50% homology. EB has been studied extensively and has been reported to promote cell proliferation and migration independently of IGF-I and its receptor (IGF-IR) in culture, but the mechanism by which EB causes these actions has not been identified. Further, the properties of EA have not been evaluated. Therefore, the goals of this thesis were to determine if EA and EB possessed similar activity in cultured myoblasts and in vivo, and if these actions are IGF-IR independent. In culture, we utilized synthetic peptides for EA, EB, and a scrambled control to examine cellular responses. For an in vivo study, we injected recombinant self-complementary Adeno-associated virus (AAVsc) vectors that express the IGF-I isoforms mutated at Valine 44 to Methionine into mouse skeletal muscles. This mutation renders the IGF-I non-functional, but the E-peptides are not affected. Both E-peptides increased MAPK signaling in culture and in vivo, which was blocked by pharmacologic IGF-IR inhibition. Although the E-peptides did not directly induce IGF-IR phosphorylation, the presence of either E-peptide increased IGF-IR activation by IGF-I, and this was achieved in part through enhanced cell surface bioavailability of the receptor. EB increased myoblast proliferation and migration and induced myofiber hypertrophy specifically in MHC2B fibers. Strangely, expression of either E-peptide decreased the number of satellite cells. The proliferation and migration effects were inhibited by IGF-IR signaling blockade and EB expression did not cause hypertrophy in MKR mice, which do not express functional IGF-IR. Thus, in contrast to previous studies, we find that E-peptide signaling, mitogenic, motogenic, and myogenic effects are dependent upon IGF-IR. We propose that the E-peptides have little independent activity, but instead affect growth via modulating IGF-I signaling, thereby increasing the complexity of IGF-I biological activity and regulation.