Colchicine treatment reveals a population of vasoactive intestinal polypeptide (VIP) positive cell bodies in the hypothalamic paraventricular nucleus (PVN) of the vasopressin-deficient Brattleboro rat. These immunoreactive perikarya in the PVN cannot be detected in colchicine-treated Long-Evans animals. When Brattleboro rats are administered vasopressin, there is a significant decrease in the number of VIP-immunopositive cell bodies in the PVN. Evidently, an alteration in the synthesis and/or release of VIP occurs in the absence of the antidiuretic hormone.