An adult type of hypothyroid myopathy is described in which, besides the slowing of voluntary movements, muscular hypertrophy and a mounding phenomenon – a sustained involuntary closure of the eyes after voluntary orbicularis oculi contraction – was observed. After some repetition of the movement, it disappeared, while local freezing again induced it. Some of the drugs employed in the symptomatic relief of myotonia rapidly reversed this phenomenon. It was, however, electrically silent and persisted unmodified for some time after the hypothyroid state was corrected. A transient structural or metabolic disturbance of the contractile muscular apparatus, which did not run quite parallel to the thyroid functional state, seemed the basis for this disturba