1. The specific radioactivity of [(14)C]glycerol released during the incubation of brown fat with [(14)C]glucose is much greater than that of the tissue lipid glycerol. 2. From a study of the release of [(14)C]glycerol from pre-labelled brown fat, it is concluded that the tissue contains a pool of glycerides with a higher rate of turnover than those in the main lipid store. 3. This pool contains newly synthesized glycerides, has a half-life of 25-30min and supplies about 25% of the glycerol liberated by brown fat. 4. Thus, a significant fraction of the total (14)C incorporated from glucose into brown-fat lipids is released as [(14)C]glycerol during an incubation.