The effects of dietary fats consisting of different fatty acids on the content of mitochondrial uncoupling protein in the interscapular brown adipose tissue were studied in rats. Sprague-Dawley male rats were meal-fed an isoenergetic diet based on either beef tallow or safflower oil for nine weeks. The gain in body weight during the experimental period did not differ between the two dietary groups. The weight of the brown adipose tissue was similar in the two dietary groups, whereas the weight of the abdominal white adipose tissue was larger in rats fed the beef tallow diet. The content of mitochondrial uncoupling protein in brown adipose tissue was lower in the beef tallow diet group than in the safflower oil diet group without differing mitochondrial mass between the two dietary groups. These results suggest that, in rats, a beef tallow diet reduces the content of uncoupling protein in brown adipose tissue, resulting in lower diet-induced thermogenesis as compared to a safflower oil diet.