Investigations of the ductless glands during the past 10 years which have required removal of the thyroid, parathyroids, thymus, spleen, suprarenals, and gonads alone, or in several combinations, have afforded us the opportunity of studying some of the effects of such insufficiencies on the remaining tissues of rabbits. The material accumulated under our immediate direction now seems sufficiently extensive to permit of certain statistical studies. In this paper we will report only the data relative to the effect of thyroidectomy, splenectomy, suprarenalectomy, gonadectomy, and combinations of these gland removals on the thymus gland as measured by its involution, persistence, or regeneration? Lymphoid tissues other than the thymus are also affected and will be referred to briefly. The data of 373 protocols have been included in this study. In sorting ,the material, protocols have been discarded for one or another of the following reasons: (1) animals with chronic infections (except snuffles), (2) animals which survived a given operation less than 2 weeks, (3) animals for which the notes of the gross or microscopic examination of the necropsy tissues were incomplete, and (4) animals on which the operation or necropsy was performed by individuals other than the authors. Of more importance than all of these factors of elimination in determining the value of such records, of 1 All operations were performed under ether anesthesia. 429 on