The existence of the pituitary body has been known since very early times. The Greeks, believed that the waste products of a certain chemical reaction in the brain flowed down the funnel-shaped infundibulum to the pituitary gland. From the gland, ducts were supposed to pass these waste products into the nasal cavity to form the " pituita" or nasal mucus. The fact that the pituitary body consists of two anatomical divisions, the anterior and posterior lobes, as well as the fact that its embryological origin is from two divergent primitive tissues, was disregarded in all pathological, physiological, and clinical work, until almost the close of the last century. Notwithstanding the numerous studies to which the pituitary has been subjected since the beginning of this century, many problems remain unsolved. It has claimed the attention of anatomists, histologists, embryologists, and physiologists because it is, as has been stated by Sir Walter Brown (1870 -1946), the "leader of the endocrine orchestra ". Studies relating to the development of the pituitary body have been numerous, although in many of them the consideration of the hypophysis has been only incidental to the other interests of the observer, and no single investigator has given a complete account of the morphogenesis of the gland throughout the whole of its development. The albino rat (Mus norvegicus) was chosen because: 1) the animal can breed well in confined quarters, 2) it has a short gestation period (21 days), 3) it has large litters, 4) the pituitary of the adult animal has been subjected to extensive cytological and experimental work, and 5) so far as it has been recorded, no investigation; except Schwind (1928), has attempted to describe the development of the pituitary in the rat. In particular, this present study is concerned with the morphogenesis of the pituitary body from the time of its first appearance during the embryonic life, until the age of sexual maturity of the animal. Special attention has been paid to the origin, topography, and cytology, as well as the qualitative and quantitative growth of its various lobes. The study also deals with some related problems such as the development of the cephalic end of the notochord in relation to the developing gland, and the development of the surrounding membranes, the nervous and blood supply, and the functional activity of the pituitary as shown by histological differentiation. The morphology of the P ituitary gland has been fully established for relatively few mammals, and for even fewer birds and reptiles, but nevertheless, recent studies (see below) of the hypophysial anatomy in several mammals have revealed marked variations in its morphology. Consequently, this study was undertaken in an attempt to trace the complete development of the pituitary body in a single mammal, namely the albino rat.