With the object of investigating the so called 'feedback' mechanism, whereby hormones from the various endocrine glands influence the hypothalamus in its regulation of the hypophysial tropic hormones, the activity of the nucleus preopticus (NPO) has been studied by means of karyometry under experimental conditions to understand the endocrine nature of the hypothalamic nuclei. (1) Oestrogen and testosterone propionate caused an increase in nuclear volume of the NPO cells, which is related to the synthesis of secretion. An increase of the nuclear volume is attributed to an increase of secretion. (2) Methylthiouracil (MTU) and thiourea bring about shrinkage in nuclear dimensions (p less than 0.001). Depletion of NSM is noticed in the hypothalamo-hypophysial neurosecretory system after MTU administration. (3) Two to five days' administration brings about depletion of NSM in the cells of the NPO. Eight to 15 days' metopiron treatment enhanced the activity of NPO cells. (4) Hypophysectomy caused shrinkage of the NPO cell nuclei, depletion of AF-stainable material in the cells of the NPO. ACTH treatment reversed these changes and brought about hypertrophy of the neurosecretory cells. (5) Neomercazole treatment and radiothyroidectomy bring about shrinkage in the NPO cell nuclei. Thyroidectomized toads when injected with thyroxine showed an enlargement of their nuclei.