1. The post-natal development of cholecystokinin (CCK)-like activity was studied in the brain and small intestine of the rat. CCK-like biological activity was measured in extracts of these tissues by an in vitro rabbit gall-bladder bioassay. 2. Immediately after birth, the brain contained very little CCK-like activity whereas the proximal small intestine contained significant concentrations of CCK-like activity. The concentration of CCK-like activity in the brain increased rapidly during the third post-natal week and reached adult values by the end of the fourth week. The development of CCK-like activity in the proximal small intestine differed from that seen in the brain. The concentration of CCK-like activity increased during the first post-natal week. After this time, however, the concentration decreased and the adult values, therefore, were lower than those found immediately after birth. This decrease in concentration resulted from failure of the total content of CCK-like activity to increase despite rapid growth of the intestine. 3. The composition of CCK-like activity in neonatal extracts was determined by gel filtration chromatography with Sephadex G50. Extracts of neonatal brain and intestine contained more than one molecular form of CCK-like activity in contrast to the single peak of activity found in adult extracts. In the developing intestine smaller molecular forms were found in addition to the single larger form found in the adult and in the neonatal brain larger molecular forms were found in addition to the CCK octapeptide found in the adult.