1. By immunoperoxidase technique, immunoreactive angiotensin II (ANG II) was located in the cell bodies of many magnocellular neurons in the supraoptic and paraventricular nuclei of the hypothalamus and their pathways to median eminence and posterior pituitary gland in the rat.
2. Like vasopressin and its neurophysin, but not oxytocin, ANG II was also found in parvocellular neurons in the suprachiasmatic nucleus.
3. Analysis of these peptides in the same magnocellular neurons reveals that ANG II is localized primarily in vasopressin cells.
4. Like vasopressin and its precursor, ANG II is deficient in homozygous Brattleboro rats with diabetes insipidus.
5. In adrenalectomized rats increases in vasopressin and its neurophysin in median eminence are associated with increases in ANG II.
6. The data suggest that the ANG II demonstrated shares antigenic determinants with the vasopressin precursor, or is regulated in a similar way to vasopressin in the same neurons.