Summary. Beta-endorphin appears to be involved in the hormonal response to suckling in some animals. The peripheral secretory patterns of β-endorphin, prolactin and cortisol were investigated in serial venous blood samples taken during suckling from eight healthy women who were breast-feeding on the third or fourth day of the puerperium. Plasma levels of prolactin and β-endorphin increased significantly during suckling reaching a peak after 20 min, levels of cortisol remained unaffected. It is suggested that the increased β-endorphin derives from an extra-hypophyseal source.