Levels of oxytocin (OT) and PRL were measured in plasma drawn before and during intermittent mechanical pump and tactile stimulation of the breast in five normal cycling women and 19 women in the third trimester of pregnancy. OT was significantly increased above baseline in response to breast stimulation in two of five cycling women and PRL increased in one of the two OT responders. In pregnant women, mean OT post nipple stimulation was significantly higher than pre nipple stimulation whereas PRL did not increase significantly. The response of OT to nipple stimulation occurred in 18 of 19 pregnant women compared to only two of five normal cycling women but the magnitude of the OT response in pregnant women was less than in cycling women or post-partum lactating women previously studied in this laboratory. In one non post-partum woman who induced lactation for the purpose of breast-feeding an adopted infant, OT and PRL were measured before and during mechanical pump and tactile stimulation before initiation of breast-feeding. OT increased during mechanical pump and tactile stimulation of the breast, as well as suckling, whereas PRL increased only in response to suckling. Levels of LH were measured in plasma every 20 min for 160 min at the following times: before initiation of breast-feeding, during induced lactation while breast-feeding, and 30 d after discontinuation of breast-feeding. Despite the development of oligomenorrhoea during the period of breast-feeding, levels of progesterone were not suppressed and LH was released in a normal pulsatile fashion.