Two hereditary muscular dystrophies similar to human progressive muscular dystrophy (P.M.D. Duchenne type) have been isolated in animals, one in mouse, the other in chicken. The decrease in the activity of glycogenolytic enzymes is similar to that observed in denervated muscle. Isozymic fetal types for several muscular enzymes have been observed as well in chicken as in man, but this fetal type may also be found in neurogenic atrophy. The release in circulation of muscle enzymes seems more specific. But the origin of the genetic lesion is still unknown. We describe here the three different theories about this problem: i.e. neurogenic, vascular, or myogenic. This last theory implies a trouble of membrane permeability.