The aim of this study was to investigate the effect of selective cholecystokinin (CCK) antagonists on the development of a preference for the mother by newly born lambs. At birth lambs received an injection of the CCK-A antagonist devazepide (0.01 or 0.1 mg/kg), the CCK-B antagonist PD135158 (0.01 or 0.1 mg/kg), or saline for the controls (1 ml/kg). No major side effects were observed in the first 4 postnatal hours except that lambs receiving PD135158 displayed more exploratory behavior towards the maternal body than lambs from the other groups. When tested in a 2-choice test situation at 24 hr of age, lambs treated with PD135158 or saline spent significantly more time near their dams than near the alien ewes, whereas lambs treated with devazepide did not show any discrimination. The effect of devazepide persisted at 48 hr of age. The use of a CCK-A antagonist, but not a CCK-B antagonist, was concluded to prevent the formation of a preferential relationship between the lamb and its mother, most probably by impairing neonatal learning.