This research was designed to 1) clarify the role of hormonal receptors in thermogenic tissues in the increased thermogenic response to norepinephrine observed in winter acclimatized small mammals, and 2) identify hormones which regulate hormonal receptors in thermogenic tissues of cold-exposed and seasonally acclimatized small mammals. Initial studies have assessed the number and certain binding characteristics of beta-adrenergic receptors of brown fat in warm-acclimated subarctic red-backed voles (Clethrionomys rutilus) by binding of (minus) 3H-dihydroalpreuolol (3H-DHA) directly to isolated membranes in suspension. Incubation of brown fat membranes with increasing concentrations of 3H-DHA (O.5nM to 20nM) for 15 min. at 37 degrees C showed that specific binding is a saturable process with approximately 210 fmol 3H-DHA bound per mg protein at apparent saturation. Half-maximum saturation occurred at 3 to 4 nM 3H-DHA binding. For data from a series of experiments on young voles acclimated to 23 plus or minus 1 degrees C the Scatchard plot was linear, Kd equal to 4.94 plus or minus 0.22, and the maximum number of binding sites Bmax equal to 266 plus or minus 11 fmol 3H-DHA bound per mg protein. These studies indicate that subarctic voles possess a high concentration of high-affinity beta-adrenergic receptors on their brown fat cell membranes. In progress are assessments of receptor number and affinity in brown fat of cold-acclimated and summer-acclimatized voles.