The isolation of an endogenous oxytocin-like substance from the jugular blood of puerperal women has been described by Coch, Fielitz, Brovetto, Cabot, Coda & Fraga (1967) and Brovetto, Olhaberry, Gioia de Coch, Coda, Fielitz, Cabot, Fraga & Coch, (1967).
This communication deals with the estimation of the loss of oxytocin by the action of plasma oxytocinase when using this method.
The inactivation of oxytocin by plasma oxytocinase depends on the day of puerperium, the temperature of the plasma and on the time of contact of the hormone with the plasma.
Most of the samples of blood were extracted between the third and fifth day of the puerperium. They were cooled and centrifuged at 0° and the time elapsed from the beginning of extraction of the sample to the inactivation of the oxytocinase was always less than 20 min. The losses of oxytocin were estimated on the basis of the oxytocinase