Specific binding sites for cholecystokinin (CCK) and substance P (SP) were detected in the brain of a marine teleost fish, the sea bass, after in vitro incubation of tissue sections with the tritiated peptides and light microscopic autoradiography. Specific binding sites for [H-3]-CCK were detected in the dorsal and ventral telencephalon, in the preoptic, tuberal and posterior hypothalamus, in the optic tectum, in the valvula cerebelli, in the vagal lobe and further in a dorsal location in the medulla oblongata. Areas rich in [H-3]-SP binding were located in the ventral telencephalon, in the entire hypothalamic and thalamic region, in the midbrain tegmentum, in the optic tectum, in the valvula cerebelli and in the medulla oblongata. The distribution of these binding sites seemed to match fairly well with the location of the corresponding immunoreactive elements, although some minor mismatches could be observed. These autoradiographic findings provide the first anatomical evidence for the presence of CCK-like and SP-like binding sites in the brain of a teleost fish.