beta-Adrenergic receptor binding on circulating lymphocytes was evaluated in young female bulimic patients (n = 12) and age- and sex-matched normal control volunteers (n = 10). Using iodine 125-labeled cyanopindolol, antagonist binding was evaluated (number of receptors [Bmax] and dissociation constant [KD]), and using isoproterenol competition of cyanopindolol binding, the concentration required to inhibit binding by 50% (IC50) for isoproterenol and the agonist affinity measure of KL/KH (ratio of dissociation constants for the low- and high-affinity states of the receptor) were determined. Plasma norepinephrine (NE) level was also measured. There was a trend toward lower plasma NE levels in the bulimic patients. The KL/KH ratio in bulimic patients was significantly greater than that for the normal volunteers, indicating increased receptor coupling. The KL/KH ratio was not significantly correlated with plasma NE level. Neither Bmax nor KD was different between the two groups. These findings suggest that beta-adrenergic receptors in bulimic patients may be more responsive than in normal subjects, without alteration of the traditional measures of receptor responses, a difference that cannot be explained on the basis of plasma NE. These findings provide another line of evidence for altered regulation of the noradrenergic system in bulimic patients during a controlled phase of their illness.