PITUITARY ABLATION by stereotactic transnasal Yttrium Y 90 implantation has been carried out at this institution in a number of patients with advanced metastatic mammary carcinoma.1 The occurrence of subsequent water intoxication and the ultimate degree of diabetes insipidus varied from case to case, indicating that the interruption of the hypothalamico-hypophyseal system was frequently incomplete.2 The question is: Are these disturbances determined by the degree of pituitary damage or by the degree of retrograde loss of hypohyponeurons, or both? In the present study an attempt was made to correlate diabetes insipidus with hypothalamic damage by assessing the degree of nuclear and neuronal atrophy occurring in the supraoptic (SON) and paraventricular (PVN) nuclei of five patients who died from 5 to 20 months following Yttrium Y 90 hypophysectomy and in whom both anterior and posterior pituitary function had been studied preoperatively and postoperatively. Estimates of degree of