Studies in which plasma osmolality was altered acutely by oral water loading and hypertonic sodium chloride infusion were performed to further identify the mechanisms involved in the pathogenesis of the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) in a patient with Guillain-Barré syndrome. Although resetting of the osmotic threshold for vasopressin release was demonstrated in these studies, this does not seem to have been a primary factor in the development of SIADH in this patient. Downward resetting of the osmotic threshold by sustained hypoosmolality has been previously demonstrated, and it is possible that this may account for the initially low osmotic threshold identified by our studies. These studies suggest that inappropriate antidiuresis, as shown by the absence of a diuretic response to low threshold suppression of the plasma arginine vasopressin concentration was due either to a vasopressin-independent mechanism or to markedly increased renal tubular sensitivity to vasopressin.