The pharmacological properties of [3H]-WB4101, [3H]-clonidine and [3H]-dihydroalprenolol binding in chick brain membranes display the characteristics known for alpha 1-, alpha 2- and beta-adrenergic binding sites, respectively. Kinetic studies performed at different embryonic and post-hatching ages have shown one binding component for each one of the above radioactive ligands. The ontogeny of alpha 1, alpha 2 and beta binding sites was studied in cerebral hemispheres, optic lobes, brain stem and cerebellum. In all brain regions studied, the development of alpha 2 binding sites precedes that of alpha 1 and beta, and a very significant decrease of alpha 2 number was observed in the cerebellum, brain stem and optic lobes afterwards. The autoradiographic localization of adrenergic receptors was studied in the optic lobes and cerebellum. In the optic lobes the superficial layers of stratum griseum and fibrosum showed a strong selective labelling of alpha 1, alpha 2 and beta binding sites and the strong selective labelling of alpha 2 binding sites extended to the layer of stratum opticum. Among the nuclei located in the optic lobe only the nucleus mesencephalis lateralis pars dorsalis (MLD) exhibited a strong selective labelling for alpha 1 binding sites while, for beta binding sites, not only the MLD, but also the nucleus isthmic pars parvocellularis (Ipc) and the nucleus isthmic pars magnocellularisa (Imc) exhibited strong labelling. In the cerebellum strong selective labelling for alpha 1 and beta receptors was seen in the molecular layer. Labelling of the granule cell layer was almost equally strong for alpha 1 but significantly less for beta binding sites. No significant labelling could be detected for alpha 2 binding sites.