T3, T4, TSH and myoglobin levels were determined in 14 patients with myasthenia gravis and compared with thirty one healthy control subjects. Significant decrease of myoglobin concentration and increase T3, T4 levels as compared to control group was found. TSH concentration was in normal range. Duration of myasthenia, level of disease (according to Osserman classification) and kind of therapy not influenced on detectable variations myoglobin, T3, T4 and TSH concentration.