Recently developed radioimmunoassay (RIA) techniques were employed in a quantitative investigation of the release and renal actions of the avian antidiuretic hormone, arginine vasotocin (AVT) in the conscious domestic fowl. The investigation was composed of a series of three studies with goals to: (1) define the functional characteristics of the avian hypothalamo-neurohypophysial system (HNS) in terms of the extracellular osmotic and volemic stimuli that elicit release of AVT; (2) characterize responses of the HNS to the natural challenge of negative fluid balance; and (3) quantify the relative contributions of AVT-induced glomerular and tubular antidiuresis to the conservation of water by the avian kideny. Characterization of AVT secretion revealed that plasma osmolality (P(OSM)) is a primary determinant of AVT secretion by the HNS of the domestic fowl. Highly correlated and significant relationships between P(OSM) and plasma AVT (P(AVT)) exist both above and below the observed basal P(OSM) of normally hydrated birds. Analysis of the present data also suggested that the HNS is insensitive to changes in blood volume of 10% or less, since neither isotonic expansion nor reduction of blood volume altered P(AVT). The results of the fluid deprivation experiments indicate that AVT secretion is closely linked to the state of hydration during negative fluid balance in the domestic fowl. Analysis of the data indicated that increases in P(AVT) that occur with dehydration are mediated primarily by extracellular hyperosmolality and that the HNS is relatively insensitive to the simultaneous hypovolemia incurred with fluid deprivation. Comparison to the dose-response relationships between P(AVT) and glomerular and tubular mechanisms of antidiuresis over the entire range of physiological P(AVT) levels in the domestic fowl revealed that tubular mechanisms are of primary importance and glomerular mechanisms of secondary importance in the conservation of water by the avian kidney. The greatest proportion of the total AVT-induced reduction in renal water excretion occurred at low physiological P(AVT) levels and appeared to be the exclusive result of tubular mechanisms of antidiuresis. At high P(AVT) levels, glomerular and tubular mechanisms overlapped and their effects on water conservation could not be separated; however, only minor additional amounts of water were conserved by the combined actions of glomerular and tubular mechanisms. Thus glomerular mechanisms appear to have only a minor, secondary effect on water conserving ability of the avian kidney.