Radioreceptor binding studies were designed to localize and determine the number of alpha- and beta-adrenergic receptors in the mammary gland of lactating cows. 3H-prazosin, 3H-rauwolscine and 3H-dihydroalprenolol were used for the regional characterization of alpha 1-, alpha 2- and beta-adrenergic receptors by competitive inhibition of binding of 3H-ligands with unlabelled adrenergic agonists and antagonists. The alpha 1-, alpha 2- and beta 2-adrenergic receptor subtypes could thus be demonstrated in the regions of the teats, large mammary ducts and parenchyma. Tissues of the teat wall, of the large mammary ducts above the gland cistern and of the mammary parenchyma were prepared to determine the density of alpha 1-, alpha 2- and beta-receptors by saturation binding assays using 3H-prazosin, 3H-rauwolscine and 3H-dihydroalprenolol respectively. Binding to high affinity sites was reversible within minutes and saturable. Equilibrium was reached within minutes. The number of alpha 1- and alpha 2-adrenergic receptors decreased from the teat to the mammary ducts to the parenchyma. Most of the alpha 1- and alpha 2-adrenergic receptors were found in the teat wall, whereas in the parenchyma alpha-adrenergic receptors were absent or barely detectable. The density of beta-adrenergic receptors was similar in the teat wall and the large mammary ducts, but much lower in the parenchyma. Thus, alpha 1-, alpha 2- and beta-adrenergic receptors were found mainly in the milk purging system and hardly at all in mammary parenchyma. Inhibition of milk removal by alpha-adrenergic stimulation is possibly due to constriction of teat wall and to constriction of the mammary ducts, whereas enhanced milk flow after beta-adrenergic stimulation is possibly due to relaxation not only of the teat sphincter and teat wall, but probably also of the large mammary ducts.