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In the present study we have attempted to answer the question whether vasopressin and oxytocin are secreted together or independently. This subject has been a matter of controversy during the past 35 years. The great variety of methods used in measuring oxytocin and vasopressin activities, the variety of animal species studied, and the variety and magnitude of stimuli employed in different studies make it very difficult to compare results and to draw conclusions as to whether or not there may be independent release of these two neurohypophyseal hormones (1-5).The studies to be described in the present report have been performed in conscious women in whom several relatively mild stimuli known to act on the neurohypophysis were applied under carefully controlled conditions. In the absence of adequate methods for assaying oxytocin and vasopressin directly in plasma, both hormones were assayed by indirect methods that have gained general acceptance. The results strongly suggest that under appropriate conditions oxytocin and vasopressin are released independently of each other by the human neurohypophysis. MethodsStudies in 29 women were carried out during the last weeks of pregnancy (36 to 40 weeks) or during the first week of puerperium, or both.In preparation for the measurement of vasopressin activity, a constant degree of overhydration was induced by the oral administration of 10 to 20 ml of water per kg of body weight or the intravenous administration of a similar amount as 5% glucose in water. Urine was collected through an indwelling catheter every 15 min-* Submitted for publication January 27, 1964; accepted August 10, 1964. Presented at the Forty-fifth Annual Meeting of the Endocrine Society, Atlantic City, N. J., June 13, 1963. Supported by grant 59028 from the Rockefeller Foundation.utes. Venous blood was collected for use in the determination of free water clearance (CH20), solute clearance (Cosm), and creatinine clearance (Ccr). Osmolality was monitored by the freezing point depression method (Fiske osmometer), and creatinine was measured by the method of Bonsnes and Taussky (6). An increase in urine osmolality (decrease in CH20) without significant changes in CCr and Cos.1 was taken as evidence of vasopressin activity (antidiuretic response) (7).In late pregnancy, oxytocic activity was assayed by monitoring intrauterine pressure by the method of Caldeyro-Barcia and associates (8, 9). A thin polyethylene catheter was inserted into the uterine cavity through the anterior abdominal wall and connected to a pressure transducer and a Sanborn Poly-Viso recorder. This procedure has, with long experience, proved to be innocuous. Uterine or "oxytocic" activity was expressed in Montevideo units, representing the product of the amplitude of the contractions in millimeters of mercury multiplied by the frequency of the contractions during a given period (8, 9). During the puerperium, oxytocin was assayed by the measurement of intramammary pressure or "milk-ejecting pressure" by the method of Sica-Blanco and co...

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