Fluorescence histochemical and immunocytochemical techniques were used to investigate morphologic correlates of the relationship between catecholamine varicosities and vasopressin-containing perikarya in an animal model of vasopressin excess, the nephrogenic diabetes insipidus mouse. Our results show hypertrophy and increased immunoreactivity in vasopressin neurons in these mice were accompanied by a marked increase in the density and to some extent the fluorescence intensity of catecholamine varicosities within the supraoptic nucleus. These results further support the concept of functional interactions between catecholamine and vasopressin neurons and raise the possibility that the target neuron, or one of its products, perhaps vasopressin, either exerts a trophic influence on the catecholamine innervation pattern of the supraoptic nucleus or enhances catecholamine content in existing fibers and terminals.