The results of this study suggest that the atypical β-adrenergic receptor (β3 subtype) is expressed in human white adipose tissue. A β3-adrenergic receptor mRNA could be detected in human omental fat poly(A)+ RNA by polymerase chain reaction amplification with appropriate primers. The expression of the β3-adrenergic receptor in human fat was confirmed by Northern blot analysis; however, the level of its mRNA was lower than those of the β1- and β2-adrenergic receptors. Two populations of ((−)-4-(3-t-butylamino-2-hydroxypropoxy)-[5,7-3H]benz-imidazole-2-one) ([3H]CGP 12177)-binding sites were identified in human omental fat membranes, one with a high affinity (Kd= 1·7 nm) and the other with a low affinity (Kd = 22 nm). The low affinity binding site population, which should represent the β3-adrenergic receptor, was predominant (75% of the total binding sites).