Transcriptional cascades that specify cell fate have been well described in invertebrates. In mammalian development, however, gene hierarchies involved in determination of cell lineage are not understood. With the recent cloning of the MyoD family of myogenic regulatory factors, a model system has become available with which to study the dynamics of muscle determination in mammalian development. Myogenin, along with other members of the MyoD gene family, possesses the apparent ability to redirect nonmuscle cells into the myogenic lineage. This ability appears to be due to the direct activation of an array of subordinate or downstream genes which are responsible for formation and function of the muscle contractile apparatus. Myogenin-directed transcription has been shown to occur through interaction with a DNA consensus sequence known as an E box (CANNTG) present in the control regions of numerous downstream genes. In addition to activating the transcription of subordinate genes, members of the MyoD family positively regulate their own expression and cross-activate one another's expression. These autoregulatory interactions have been suggested as a mechanism for induction and maintenance of the myogenic phenotype, but the molecular details of the autoregulatory circuits are undefined. Here we show that the myogenin promoter contains a binding site for the myocytespecific enhancer-binding factor, MEF-2, which can function as an intermediary of myogenin autoactivation. Since MEF-2 can be induced by myogenin, these results suggest that myogenin and MEF-2 participate in a transcriptional cascade in which MEF-2, once induced by myogenin, acts to amplify and maintain the myogenic phenotype by acting as a positive regulator of myogenin expression.The formation of skeletal muscle during vertebrate development involves the induction of mesoderm from primary ectoderm and the subsequent generation of proliferating myoblasts that ultimately terminally differentiate in response to environmental cues. The recent discovery of a family of related muscle-specific factors that can convert fibroblasts to myoblasts has contributed to rapid progress toward understanding the molecular events that underlie the establishment of the skeletal muscle phenotype (for reviews, see references 55 and 69). Members of this muscle regulatory gene family include MyoD (21), myogenin (25, 77), myf5 (9), and MRF4/herculin/myf6 (8,48,61), each of which can activate myogenesis when introduced into a wide range of nonmuscle cell types.