Concentrations of oxytocin have been measured in plasma samples obtained daily from 6 women throughout complete menstrual cycles. Measurements of menstrual blood loss and plasma levels of luteinizing hormone (LH), progesterone and 17.beta.-estradiol suggested that normal ovulatory cycles had been studied. A significant elevation in the circulating concentration of oxytocin was found in association with peak levels of LH. Oxytocin may have a role in the process of ovulation in the human female.