Lesions were made by electrical coagulation in the base of the hypothalamus of rats, 20 to 24 h after birth. Subsequent growth of the hypophysis was examined volumetrically and histologically on the 40th and 60th days after the operation, paying special attention to the pars intermedia. Growth retardation of the hypophysis was noticeable after 60 days in rats with lesions of the hypophysial stalk (groups 1 and 2) and the posterior portion of the median eminence (group 4), especially in animals in which the stalk was completely (group 1) or almost completely (group 2) transected. Notwithstanding the growth retardation of the hypophysis, the ratios of the volume of the pars anterior to that of the whole hypophysis in groups 1 and 2 were not significantly different from those in group 3 (animals with a lesion of the anterior portion of the median eminence), groups 4 and 5 (animals with a lesion in the hypothalamus elsewhere than the stalk and median eminence), and normal controls of the same age (group 6). In contrast to this, the ratio of the volume of the pars nervosa to the total gland in group 3, and more especially in groups 1 and 2, had considerably diminished, and that of the pars intermedia in groups 1 and 2 had markedly increased, after 60 days in comparison with those in groups 3, 4, 5, and 6. These results indicate that the pars intermedia of animals in groups 1 and 2 was hypertrophic compared with the pars anterior and pars nervosa. These volumetric data seem to correspond with the finding that neurosecretory material was not present in the pars nervosa in groups 1 and 2, but was abundant in groups 3, 4, 5, and 6. After 60 days, the pars intermedia of group 1 consisted chiefly of well-developed cells, as in normal adult rats. On electron microscopic examination, however, they were found to differ from normal mature cells in the following three points: (1) most of the secretory granules were vesicular with little or no opacity; (2) the endoplasmic reticulum frequently showed cisternae; (3) nerve terminals were not encountered at the surface of the glandular cells. The former two points suggest that these cells are functionally active, an observation that agrees with the results of volumetric findings. It is concluded from these data that, in the absence of hypothalamic control, the pars intermedia of the hypophysis in rats grows actively after birth, while the anterior hypophysis seems to be inactive and atrophic [unpublished].