Arginine vasopressin (AVP) and vasotocin (AVT) were measured by radioimmunoassay in extracts of cerebral cortex, cerebellum, brain stem, pineal, hypothalamus, and neurohypophysis from normal Long-Evans rats. AVP was present in expected concentrations in pituitary and hypothalamus and there was no evidence of its accumulation elsewhere. AVT was not detectable in these tissues (within the limits imposed on our assay by the presence of excessive amounts of AVP) but was easily detectable in pineal tissue with a concentration of 22.4 ± 6.6 µU/gland. Extracts of neurohypophysis and pineal glands from homozygous Brattleboro rats (rats with hereditary hypothalamic diabetes insipidus) revealed the total absence of AVP and AVT. We conclude that the Brattleboro rat is incapable of synthesizing biologically active neurohypophyseal peptides which contain arginine in position