A pituitary stalk-section rat model was prepared that would be suitable for the study of the direct effects of steroids on the pituitary gland. In this model, the hypophyseal stalk was severed surgically and metal foil was inserted to prevent regeneration of portal vessels. The major portion of the pituitary no longer had a direct communication with the hypothalamus, and the only blood supply was from short portal vessels that supply the dorsolateral part of the gland. The effectiveness of the lesion was demonstrated by very high levels of serum prolactin and very low FSH and LH. Light microscopic examination revealed a large infarct in the central region of the gland with stainable cells in the peripheral area. Fourteen days after stalk-section, the infarct had shrunken appreciably, and after 21 days it was replaced by a scar. Ultrastructural studies revealed the presence of functional mammotropes in the peripheral regions of the pituitary gland, while other cells were smaller, less active, and contained fewer secretory granules. After LHRH administration there was an increase in serum LH, thus indicating the presence of a substantial number of viable gonadotropes. This was confirmed by the ultrastructural findings which now showed the appearance of large, well-granulated gonadotropes. When LHRH was given to estradiol 17-beta primed ovariectomized stalk-sectioned rats, there was a great augmentation of LH release, and many castration cells were now apparent. Therefore, the administration of estrogen and LHRH was needed for the gonadotropes of the isolated pituitary to function optimally in the synthesis and secretion of FSH and LH. The stalk-sectioned animal model permits evaluation of pituitary function isolated from hypothalamic control while it leaves the surviving pituitary gland in its normal anatomical position.