Desmin, the muscle-specific intermediate filament protein is one of the earliest known myogenic makers both in heart and in somites. We have previously shown that high levels of desmin expression in the skeletal cell line C2C12 are due to a distal enhancer, which contains a muscle-specific factor-(2MEF2) binding site, adjacent to an E box, the binding site of the myogenic Helix-Loop-Helix (mHLH) regulators. We have further shown that MEF2C, a myocyte restricted member of the MEF2 family and all four mHLH factors can bind to their corresponding sites and through a cooperation with a second proximal E box can transactivate the desmin promoter. To study the significance of these regulatory elements in vivo, we have generated transgenic mice with desmin-lacZ reporters, intact or mutated at the MEF2 and E box of the enhancer. We show that the cis-acting DNA sequences within the 1-kb 5' flanking region of the mouse desmin gene are sufficient to direct appropriate temporal transcription both in heart and in skeletal muscle during mouse embryogenesis. Mutation at the MEF2 site completely suppressed transcription of the linked lacZ transgene in both developing heart and somites of the embryos. Mutation of the E box only suppressed activation in skeletal muscle precursors (somites and limb buds) but not in cardiac muscle. These data demonstrate that the MEF2 site is indispensable for the desmin enhancer function both in heart and in skeletal muscle. In addition, MEF2 cooperation with the mHLH regulators is absolutely necessary for proper transcriptional activity during embryonic skeletal muscle development.