We introduced the radioimmunoassay (RIA) of arginine vasopressin (AVP) with standard AVP and antiserum to AVP (both Calibiochem). The sensitivity of the system was increased from the declared 4pg to 1 pg per tube by preparing AVP-125I of high specific activity (about 1,500 mCi/mg) and by modifying the reaction conditions. The sensitivity of the method was adequate for measuring AVP in urine and in concentrated plasma extracts, even under physiological conditions. Reliability of the results depended upon maintenance of approximately the same osmolarity in all the RIA samples. The mean plasma AVP level, uncorrected for AVP extraction losses, was 1.52 +/- 0.20 pg/ml for an ad libitum fluid intake; in fluid deprivation it rose in proportion to the osmolarity of the plasma to 5.83 +/- 0.42 pg/ml at 12 hours and to 19.09 +/- 4.51 pg/ml at 36 hours. Extraction recovery of added AVP was about 63%. The urinary AVP concentration varied according to the patients' state of hydratation from undetectable values at UOsm less than 200 mOsm/1 to a mean 16.5 +/- 7.9 pg/ml in the presence of an ad libitum fluid intake and to 29.1 +/- 7.5 pg/ml after 12 hours' and 117.2 +/- 13.7 pg/ml after 36 hours' deprivation of fluids.