A radioimmunoassay specific for arginine-vasopressin (AVP) developed in recent years has been applied to measurement of urinary AVP during physiological and clinical studies. Daily excretion of AVP was 34 +/- 10 ng (mean +/- SD) in 17 female normals and 69 +/- 45 ng in 17 male normals, this being a significant difference (p less than 0.01). After osmolar load (Carter-Robbins test) hourly excretion of AVP increased significantly in 7 males from 1.3 to 3.1 ng/h and in 6 females from 1.7 to 6.5 ng/h. Again the difference between male and female normals was significant. In both sexes a significant correlation between AVP excretion and either plasma osmolality or free-water clearance was found. When the osmolar load test was performed under constant angiotensin II perfusion in male subjects, their AVP excretion was significantly more elevated; this confirmed in man the hypothesis that angiotensin II is a stimulus to AVP secretion. Preliminary results of AVP excretion and response to osmolar load in diabetes insipidus are reported. Exceedingly high rate of excretion of AVP up to 55 330 ng/24 h have been found in cases of bronchial carcinoma with dilutional hyponatremia.