A radioimmunoassay for human plasma vasopressin (AVP) which permits the estimation of antidiuretic hormone (ADH) level as low as 0.8pg/ml, was developed. The average plasma level of AVP after overnight water restriction was found to be 14.3pg/ml (sd=4.4pg/ml) in normal subjects. They provoked a hypersecretion of ADH by the intravenous injection of 1-2mg of nicotine. In 11 volunteer normal subjects this stimulation by nicotine provoked ADH hypersecretion which reached a maximum between 2nd and 15th minutes after injection. In 3 cases of diabetes insipidus, nicotine injection did not induce ADH hypersecretion: in 1 case of potomania this response was weak; in 2 cases of syndrome of inappropriate ADH secretion, AVP plasma levels were elevated and the response after nicotine stimulation was exaggerated.