It appears likely that the blood concentration of arginine vasopressin in man in normal water balance is less than 1·5 μ-u./ml. (Bisset & Lee, 1958; Moran, Miltenberger, Shuayb & Zimmermann, 1964). Any attempt to assay this small quantity requires a very sensitive preparation, or a method of extraction which will concentrate the hormone with little loss, or both. Heller & Štulc (1959) have reported that if the urinary bladder was exteriorized several days before using rats for assay and if a needle was inserted into the tail vein on the day of the experiment, the rat would respond to 0·625 μ-u. of vasopressin, an increase of sensitivity about fivefold greater than previously reported. Czaczkes, Kleeman & Koenig (1964) confirmed these results using female Sprague-Dawley rats weighing 100–120 g.; a water-ethanol load was maintained by infusion into the tail vein. Urine flow was measured every 10 min. We have followed this procedure