The binding characteristics of the beta-adrenergic receptor in the rat ventral prostate homogenate have been studied using the highly potent beta-adrenergic antagonist [125I]cyanopindolol (CYP) as ligand. The bound ligand was separated from the free moiety by precipitation with polyethylene glycol (PEG-6000). This technique is simple, accurate, fast and more advantageous than filtration of the hormone-receptor complex on glass fiber filters or direct centrifugation. [125I]CYP binds to a single class of high affinity sites at an apparent KD value of 23 pM. Using 0.1 microM (-)propranolol to determine non-specific binding, a number of sites of 600 fmol/mg protein were measured. The observed order of potency of adrenergic agonists (KD values) in competing for [125I]CYP binding was: (-)isoproterenol (25 nM) greater than (-)epinephrine (74 nM) much greater than (-)norepinephrine (1900 nM). Detailed study of the binding potency of a large series of beta 1- and beta 2-adrenergic agonists and antagonists showed the presence of a typical beta 2-subtype adrenergic receptor in the rat ventral prostate. The best estimate indicates that the proportion of beta 2-adrenergic receptors in rat ventral prostate is more than 95% of the total population of beta-adrenergic receptors in this tissue. The high selectivity and density of beta 2-adrenergic receptors in rat ventral prostate suggest a physiological role of circulating and/or locally secreted catecholamines in the control of prostatic growth and function.