The influence of insertion of a needle into muscle on the activity of creatine kinase and fructose-1,6-diphosphate aldolase in serum was measured in 23 patients undergoing electromyography because of suspected neurogenic (n = 18) or myogenic (n = 5) muscular atrophy. Myogenically atrophied muscle was easily vulnerable. In neurogenic atrophy and in healthy muscle the lesion was demonstrable in enzyme changes only when muscular work followed the insertion of the needle. Activity of so-called muscle enzymes is not only changed after intramuscular injections but also due to insertions of thin electrodes (electromyography). The vulnerability of muscules may provide information on the activity of an atrophying process.