Plasma oxytocin concentrations were measured during 12 minutes of mechanical pump stimulation in nine healthy women during the follicular and luteal phases of the menstrual cycle. In the follicular phase (and in five healthy male subjects), plasma oxytocin values did not increase above baseline levels with breast pump stimulation. In contrast, pump stimulation evoked a significant increase in plasma oxytocin levels during the luteal phase of the menstrual cycle. These observations suggest that oxytocin secretion is modulated by hormones involved in the menstrual cycle in women.