The effect of sulfhydryl reagents on binding of the β-adrenergic antagonist (−)-[3H]dihydroalprenolol hydrochloride ((−)-[3H]DHA) to a microsomal fraction of rabbit ventricular muscle was studied. Incubation with the disulfide reducing agents dithiothreitol (DTT), 2-mercaptocthanol, and reduced glutathione resulted in loss of (−)-[3H]DHA binding. At 500 μMDTT, less than 50% of specific binding activity remained; at 100 mM, binding was completely eliminated. 2-Mercaptoethanol and reduced glutathione were less effective than DTT at inhibiting binding activity. The total binding capacity (Bmax) decreased from 155.4 fmol mg−1of protein, in the absence of DTT, to 92.4 and 77.5 fmol mg−1at 0.25 and 0.7 mMDTT, respectively. The equilibrium dissociation constant (KD) increased from 7.6 nM, in the absence of DTT, to 10.3 nMat 0.25 mMDTT and to 20.8 nMat 0.7 mMDTT. Thus, DTT-induced decline in (−)-[3H]DHA binding results from a decrease in both the number and affinity of membrane binding sites for the tracer. Receptors could be protected from DTT inactivation by preincubation with β-adrenergic ligands. Oxidants could not reverse inactivation, with the exception ofo-iodosobenzoate which was only partially effective. Thus, the β-adrenergic receptor of rabbit ventricular muscle contains essential disulfide moietie(s) which can be inactivated by reducing thiols.