It has been shown that the injection of anterior pituitary extracts into thyroidectomized guinea pigs results in a marked and sustained exophthalmos (Marine and Rosen, '34 ; Smelser, '36; Paulson, '37; Aird, '41) which is strikingly similar to that found in man, particularly in those cases which persist or progress after thyroidectomy. The orbital fat and muscles of such animals hypertrophy and are edematous as in the clinical cases (Smelser, '37). The exophthalmos and the orbital changes are not produced by neural stimuli passing through the cervical sympathetic ganglia or by contraction of the smooth orbital muscle of Riliiller (Smelser, '39 a) -factors which are minimal or not involved in the human. I n addition to the hypertrophy of the orbital fat and muscles there is an increase in weight (Ca 40%) of a large orbital gland (Harder's) under the influence of injected anterior pituitary extracts. Because of its size and retrobulbar position the hypertrophy of Harder's gland undoubtedly contributes to the exophthalmos produced by anterior pituitary extract injection. Therefore, it seemed desirable to determine whether or not this type of exophthalmos was dependent upon the presence of this orbital gland or whether hypertrophy of f a t and muscles alone could be responsible for the proptosis.