The Ca2+-ATPase (SERCA) found in vesicles derived from the sarco/endoplasmic reticulum vesicles of rats brown adipose tissue and rabbit white muscle were identified by gel electrophoresis, Western blot, electron microscopy and immunolabeling with gold particles. In both tissues, the isoform found was SERCA 1. The Ca2+ affinity of the fat SERCA 1 was different from the muscle isoform. The degree of uncoupling is estimated measuring the ratio between Ca2+ transport and ATP cleaved. In brown fat vesicles the degree of uncoupling varied depending on the Ca2+ concentration of the medium. This was not observed in vesicles derived from muscle. At all Ca2+ concentrations tested, the uncoupling was not related to Ca2+ leakage from the membrane and was far more pronounced in fat than in muscle vesicle. When a Ca2+ gradient was formed across the vesicles membrane the heat released during ATP hydrolysis varied between 22 and 26 Kcal/mol in both fat and muscle vesicles but in the absence of a gradient the heat released was 17 Kcal/mol in fat and 12 Kcal/mol in muscle. The data reported indicate that the SERCA 1 of brown adipocytes is far more thermogenic than the white muscle SERCA 1, and suggest that, in addition to storing Ca2+ inside the endoplasmic reticulum, the SERCA 1 may represent a source of heat production contributing to the thermogenic function of brown adipose tissue.