Research on the neuroendocrine control of maternal behaviour has concentrated on the role of ovarian and pituitary hormones (1). It is known that the gastrointestinal tract plays an important role in synchronizing mother-young interactions (2), but the possible contribution of gastrointestinal secretions to maternal behaviour has not been investigated. We show here that treatment with oestradiol benzoate (OB) in combination with cholecystokinin octapeptide (CCK-8), a duodenal peptide (3), stimulates maternal behaviour within 4 h of exposure to newborn pups in ovariectomized rats. The elevated concentrations of CCK-8 which are found in the plasma of lactating rats may, therefore, contribute to the development and maintenance of mother-young interactions during lactation.