Tissue ir-dynorphin has been shown to be associated with three components separable by gel filtration. Similar components were found in both pituitary and brain, although there are differences in the relative amounts of each component in each tissue. Anterior lobe of rat pituitary contains predominantly the component with highest apparent molecular weight, while the component with smallest apparent molecular weight, which is the major form in neurointermediate lobe, was absent. Exposure of rats to NaCl resulted in a parallel depletion of ir-dynorphin and ir-[Arg]-vasopressin neurointermediate lobe stores. Thus a role for dynorphin, acting in concert with vasopressin, or in the regulation of vasopressin release, appears possible.