This is a detailed presentation of original work by the author on the effects of hypophysectomy and castration on various functions in the rat. These were studied chiefly through the agency of parabiosis. The technics of hypophysectomy and parabiosis are considered at length; that of the latter is illustrated by reproductions of photographs. Many photomicrographs showing histologic changes in various organs are included. Three really beautiful color plates illustrating normal and pathologic cellular detail of the pituitary are appended. Endocrinologists will find this a valuable acquisition to their libraries.