Elevation of Oxytocin and the Oxytocin-Associated Neurophysin in the Plasma of Normal Women during Midcycle*

A positive correlation was observed between the midcycle elevation of estrogen (E) and the level of oxytocin- and estrogen-stimulated neurophysin (ESN), the protein carrier of oxytocin, in the plasma of five of six women. The time of the maximal level of E was associated with a level of oxytocin significantly greater than that in either the early follicular or late luteal phase (P less than 0.025). Likewise, the level of ESN at midcycle was greater than the level of ESN in the early follicular or late luteal phase (P less than 0.01). Other than states of lactation or pregnancy, this is the only described cyclic secretion of oxytocin in humans. Since oxytocin chronologically correlates with a rise in the level of E at midcycle, a role for oxytocin in ovulation may be considered.