We have characterized and quantified the binding of [3H]CGP-12177 to beta-adrenergic receptor sites in slices (300 microns) of rat cerebral cortex. The receptors are stereospecific, saturable and of high affinity. Binding of [3H]CGP is readily reversible and demonstrates appropriate drug specificity. This assay method allows the demonstration of isoproterenol-induced down-regulation (internalization) of beta-adrenoreceptors. Receptor recycling is observed at 37 degrees C in the absence of beta-agonist but can be blocked by low temperature (0 degree C) or by monensin. beta-Adrenoreceptors can also be labeled and quantified in intact, single pineal glands of rat, mouse and hamster. Rat pineals contain approximately 10 times more binding sites than do hamster or mouse pineals and up to 8 times more sites than found in rat cerebral cortex. Rat pineal [3H]CGP binding can be up- and down-regulated but not to the same degree as seen in brain slices. This assay method is simple, rapid and provides new opportunities for the study of other receptor types in intact tissue.