The concept that secretion of the antidiuretic hormone, arginine vasopressin, is regulated by the osmolality of body water originated with the pioneering studies of Verney over 25 years ago [1]. In a classical series of experiments, this investigator showed that the injection of hypertonic saline into the carotid artery of conscious, hydrated dogs resulted in a prompt and reversible fall in urine output that was indistinguishable from that produced by the i.v. injection of small amounts of pituitary extract. On the basis of this and other indirect evidence, Verney concluded that the release of vasopressin from the neurohypophysis was controlled by an intracranial osmoreceptor that was extremely sensitive to changes in the blood concentration of sodium and certain other solutes.Efforts to confirm and extend these observations were limited for many years by the lack of a suitable method for measuring vasopressin directly at the low concentrations normally present in body fluids. Recently, however, several laboratories have succeeded in developing sensitive and specific radioimmunoassay methods that permit the hormone to be measured at physiologic concentrations with greater ease and precision [2,3]. As reviewed in the following pages, these new assay methods now have made it possible to begin to characterize osmoregulatory function in a more concrete and comprehensive way and also to use such information to analyze systematically certain clinical disorders of salt and water balance.