The retina represents a part of the central nervous system (CNS) with a well studied, geometrically defined structure and a specialized function--the processing of light signals. Neurotransmitters such as glutamate, aspartate, glycine, gamma-aminobutyric acid (GABA), dopamine and acetylcholine are considered to be involved in the neuronal activity of the retina. Receptors for acetylcholine, GABA, dopamine and benzodiazepines have also been demonstrated. Thus, the retina can be considered as a model for the study of neuronal processing in general, in which the input, light signals, can be selected and regulated in a defined way. We report here that alpha-adrenergic receptors in the bovine retina have been characterized using radioreceptor assays. 3H-phentolamine (an alpha 1- and alpha 2-antagonist), 3H-clonidine (a preferential alpha 2-agonist) and 3H-WB 4101 [(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane, an alpha 1-antagonist] were used as radioligands, and WB 4101 and prazosin (alpha 1-antagonists), tolazine (an alpha 2-antagonist), yohimbine and its stereoisomers rauwolscine (preference for alpha 2-receptors) and corynanthine (preference for alpha 1-receptors), were used as inhibitors of radioaligand binding. Only alpha 2-adrenergic receptors were found.