The novel brown adipose tissue (BAT) selective β-adrenergic agonist, BRL 37344, is 31-fold more potent than (−)-isoproterenol in stimulating the respiratory rate of interscapular BAT fragments. BRL 37344 is also more potent (9-fold) than (−)-isoproterenol in stimulating adenylate cyclase activity of IBAT purified plasma membranes whereas, in the same preparation, it is 81-fold less potent than (−)-isoproterenol in competition displacement studies with the β-adrenergic ligand, [¹²⁵I]cyanopindolol. We have previously demonstrated that the photoaffinity reagent [¹²⁵I]cyanopindololdiazirine selectively labels a 62 kDa protein in IBAT plasma membranes that displays pharmacological properties of a β₁-adrenergic subtype. Relatively high concentrations of BRL 37344 (10 μM) are required to displace [¹²⁵I]cyanopindolol-diazirine binding to the 62 kDa protein. Taken together, the results suggest that two different populations of β-adrenergic receptors may co-exist in BAT plasma membranes: a small population (about 15 %) of atypical β-receptors and a large population of β-receptors that exhibit high and low affinities for BRL 37344, respectively.