Cholinesterase and β-naphthyl acetate esterase activities were determined in whole muscle homogenates of normal subjects and of patients with various muscular dystrophies and related myopathies. There was a moderate increase in muscle cholinesterase activity with respect to acetylthiocholine hydrolysis only in Duchenne muscular dystrophy. Cholinesterase and β-naphthyl acetate activities were essentially normal in the muscle from patients with myotonic dystrophy, other forms of muscular dystrophies, polymyositis and from one patient with myasthenia gravis. Muscle cholinesterase activity with respect to butyrylthiocholine and β-naphthyl acetate esterase activity were not markedly altered in Duchenne muscular dystrophy. Serum cholinesterase levels were normal in Duchenne muscular dystrophy and other dystrophies and in a few cases of polymyositis and spinal muscular atrophies.