The modifications of stapedial reflex have been studied in 30 subjects affected by myasthenia gravis. The test was performed in basal conditions (i.e. at least 6 h after the last administration of an anticholinesterase drug), after the injection of Tensilon and again in basal conditions, 1 week after thymectomy. The reflex was elicited in 27 patients but in 1 case it appeared only after therapy. Among the various parameters analyzed, only rise velocity, decay velocity and amplitude have shown lower values in patients than in the normal group. These parameters increased significantly after the pharmacological test and 1 week after thymectomy in 60% of the cases. The data suggest that patients with more evident muscular damage are more likely to present a rise of stapedial strength after therapy. The efficiency of thymectomy has also been demonstrated.