Binding of (-)-[3H]dihydroalprenolol [( 3H]DHA) to isolated brown adipose tissue (BAT) microsomal membranes was used to estimate beta-adrenoreceptor density, and this was found to be saturable, reversible, and stereospecific. Scatchard analysis indicated a single class of binding sites of equilibrium dissociation constant 2.2 nM, with a maximum number of binding sites of 160-170 fmol/mg protein. These values were unaffected by the age or sex of the animals, and similar Kd values were obtained for binding to heart and lung membranes. The kinetically derived Kd from forward and reverse reactions was 1.4 nM for BAT. Hofstee analysis of displacement of [3H]DHA produced linear plots for propranolol but curvilinear plots for atenolol and ICI 118,551. Atenolol showed 50-fold selectivity for beta 1-receptors and ICI 118,551 50-fold selectivity for beta 2-receptors in heart, lung, and BAT membranes. These plots indicated beta 1:beta 2-receptor ratios of 59:41 for BAT and heart and 25:75 for lung. It is possible that both beta-adrenoreceptor subtypes in BAT may be involved in the activation of thermogenesis in the tissue.