Partial or complete hypophysectomy often results in regeneration and reorganization of the cut end of the pituitary stalk to form a neurohypophysis-like organ, which has the essential components for neurohypophyseal function, namely, the axonal endings for the storage of the neurosecretory material and the blood vessels for the systemic distribution of the stored products. The degeneration and numerical reduction of the neurosecretory neurons of the nucleus preopticus (NPO) post hypophysectomy seem to depend on the state of reorganization of the severed hypophyseal stalk. In response to X- or γ-irradiation the neurons of the NPO exhibited varying degrees of degeneration; and the pituitary sometimes lost their lobular architecture with heavy infiltration of connective tissue. In specimens treated with large doses of radioiodine, several neurosecretory neurons showed hypertrophy and degranulation.