The histological picture(2,3), low basal metabolism(5), inability of thyroid to accumulate iodine, and low PBI values shown in present study, indicate a myxedematous state in dwarf mice. Daily rat pituitary transplantations into dwarf animals restore thyroid to normal (2), from which is concluded that a pituitary factor, or factors, is responsible for the poor state of the dwarf thyroid. Thyrotropin is able to stimulate the thyroid gland of dwarf mice, and sensitivity seems greater than in normal siblings. It may be concluded that there is a lack of both thyrotropin and of somatotropin in the dwarf mouse. These mice are in a state of secondary myxedema and an important etiological factor for this disorder is thyrotropic pituitary hormone deficiency.