Adipose tissues and other tissues of the pig have been examined for the presence of the mitochondrial "uncoupling protein," characteristic of brown adipose tissue, in order to assess whether brown fat is present in this species. Mitochondria were prepared from various tissues and the proteins separated on the basis of molecular weight by sodium dodecyl sulphate--polyacrylamide gel electrophoresis. Immunoblotting procedures were then used to probe for uncoupling protein, employing a rabbit anti-(rat uncoupling protein) serum. Pigs were examined at 4 days, 4 weeks, and 8 weeks of age. No evidence for the presence of uncoupling protein was found at any of these ages. The protein was, however, readily detected in brown adipose tissue from rats, mice, golden hamsters, guinea pigs, Richardson's ground squirrel, and lambs. An additional group of pigs was acclimated to the cold (10 degrees C) for a period of 10 days prior to the examination of tissues, but again uncoupling protein was not detected in any tissue. These results indicate that uncoupling protein is either absent from adipose tissues of the pig or is present at such a low concentration that it is unlikely to support thermogenesis. It is concluded that the pig does not contain adipose tissue that is functionally "brown;" adipose tissues in this species appear to be exclusively "white."