It is well known that in studying the effects of hypophysectomy, removal of the pituitary must be essentially complete without injury of the neighboring region of the brain. We devised a new method of total hypophysectomy using microsurgical instruments, and observed the effects on the gonad in the dog. 1. Operative method. The animals were anesthetized with pentobarbital sodium 30 mg/kg body weight and then a wooden bar, about 5 cm in diameter, was inserted in the mouth to displace the mandibular angle anteriodownwardly. After aseptic draping at operative field vertical incision from the midline of the vertex to just behind the mandibular angle was made on the right side of frontotemporal region. Temporal muscles were also incised vertically and retractor was applied to expose the frontotemporal bone. Following wide craniectomy down to the base of middle cranial fossa and careful opening of the dura mater, temporal lobe was retracted upward and medially at the tip of the middle cranial fossa with narrow brain retractor. As the 3rd cranial nerve and internal carotid artery were exposed, arachnoid membrane was torn with microforceps and microscissors, and aspirated the cerebrospinal fluid slowly to obtain wider exposure, then elevated posterior communicating artery to expose the pituitary body and stalk. The stalk was clipped or crushed with microforceps and then sectioned with microscissors, then pituitary body was isolated with microelevator and removed in a piece with microforceps or sucked out with suction. The wound was closed in layers. 2. Postoperative management and the results. Among these experimental animals, 6 dogs were observed for the period of 43-77 days, and in 6 dogs dexamethasone was given intramuscularly for one or two weeks following hypophysectomy, the rest were used as normal control. Following clinical observation, these animals were sacrificed with intracarotid arterial infusion of 0% neutral formalin solution and removed the brain and the gonads, and fixed in 10% neutral formalin solution. The removed brain was examined and pituitary fossa was also scrutinized. The gonads were stained with hematoxylin-eosin. Periodic acid Sciff(PAS) and toluidine blue staining methods. The observed results were as follows: 1) Average diameter of testicular seminiferous tubules was 0.17 mm in normal control group. but became markedly reduced to 0.10 mm following hypophysectomy. With administration of dexamethasone for 1 and 2 weeks, diameter ranged 0.09 mm and 0.13 mm, respectively. 2) Testicular lesions in hypophysectomy group were characterized by marked reduction of spermatogenic activity, representing stage of germ cell aplasia with loss of spermatogonia, spermatocytes and spermatids. But in cases with dexamethasone administration, the pattern resembled various stages of germ cell arrest as with increased mitotic activities of spermatogonia and spermatocytes. 3) Those numerical alternations of spermatogenic cells accompanied secondary histological manifestations which included relative increase of Sertoli cell, mild thickening of basement membrane and Leydig cell hyperplasia. The findings following administration of dexamethasone became modified to reveal less prominent Sertoli cells and rather distinct Leydig cell clustering. Basement membrane thickening remained unchanged. 4) Ovarian changes in hypophysectomized dogs on the other hand, disclosed reduction of overall size accompanying decrease in number and size of Grafian follicles, atrophy of follicular and cortical stremal cells, degeneration of cva and absence of corpus luteum. Follicular and stromal cells appeared responded to administration of dexamethasone relatively in considerable degree. 5) Mast cells normally distributed mainly in the tunica albuginea of both testis, and ovary, increased following hypophysectomy to extend along fibrotic interstitial tissue of testis, but became reduced to the level of normal control group following administration of dexamethasone.