A method is described for the extraction and concentration of oxytocin from urine, satisfactory for the radioimmunoassay of this peptide. The recovery of added oxytocin was 64·8 ± 12·6 (s.d.)% from urine samples with an osmolality less than 770 mosm./kg, and 44·9 ± 13·5% for urine samples of greater osmolality. Infusions of oxytocin at a rate of 1 mu./min into male and female volunteers showed a direct relationship between the volume of urine and the total amount of oxytocin excreted during any 1-h period. Extracts of normal male urine contained an immunoreactive material which behaved identically with synthetic oxytocin in two systems of chromatography.