Two populations of [3H]CGP 12177 binding sites exist in rat interscapular brown adipose tissue (IBAT) plasma membranes. The majority of binding sites are of low affinity with a Kd of 31 nM, a value in close agreement with that for the Kd of [3H]CGP 12177 binding to a cloned rat beta 3-adrenergic receptor (AR) expressed in CHO cells (44 nM). Competition binding studies demonstrate that the Ki values of the cloned rat beta 3-AR and of the low affinity sites in IBAT are 45 and 29 nM, respectively, for BRL 37344 and 1.4 and 1.0 microM, for (-)-propranolol. These findings strongly suggest that the low affinity [3H]CGP 12177 binding site measured in IBAT plasma membranes represents the atypical beta 3-AR in this tissue.