The postnatal ontogeny of cholecystokinin receptors and cholecystokinin was determined in rat brain. The binding of cholecystokinin (CCK) to rat forebrain receptors was very low at 1-2 days of age, rose to a maximum at 12 days (Bmax = 31 fmol/mg protein, Kd = 1.47 nM), and declined to adult levels by 26 days (Bmax = 17 fmol/mg protein, Kd = 1.39 nM). In contrast, forebrain concentrations of CCK measured by radioimmunoassay rose monotonically through day 27. Possible implications of the transient developmental peak of CCK receptors are discussed.