After treatment of rats with 5 x 0.5 mg/kg/d reserpine in membrane preparations from the parotid gland no increase in the total number of beta-adrenoceptors (Bmax) was observed using the antagonist ligand (-)3H-dihydroalprenolol. There occurred, however, a pronounced increase of the high affinity agonist binding component. Thus, it appears that not the absolute number of beta-adrenoceptor sites but the relative amount of high affinity sites is most sensitive against the sensitization process. The results are in contrast to those of other authors. We suspect that increases of Bmax-values after reserpine can be simulated by the loss of noradrenaline whereas in control membranes noradrenaline is still bound to the adrenoceptors thus preventing the radioactive ligand from access to these sites. We highly recommend to use preincubation methods prior to ligand binding studies in order to remove endogenous ligands still bound to the high affinity site of the beta-adrenoceptor.