Introduction
There is no doubt that in breast-feeding women, suckling with its associated hyperprolactinaemia prevents the resumption of ovarian activity for prolonged periods (see McNeilly, 1979). The extent of this suppression varies greatly among species but in all for which there are adequate data it appears to depend critically upon the intensity of the suckling stimulus (Lamming, 1978). Our recent data from women show that in the pattern of suckling, frequency and duration, throughout the day, are both key factors in maintaining the elevation of basal levels of prolactin associated with lactation (McNeilly, Howie & Houston, 1980a; Howie & McNeilly, 1982). Suckling also releases large quantities of prolactin, maintaining a physiological hyperprolactinaemic state which is directly associated with the duration of lactational amenorrhoea (Delvoye, Badawi, Demaegd & Robyn, 1978; Duchen & McNeilly, 1980). The question remains, how does suckling suppress ovarian activity?
The levels of prolactin in blood during peak
