In adult man, brown fat can be detected in perinephric fat depots by visual inspection, electron microscopy and nucleotide binding to the tissue-specific uncoupling protein. The 32 kDa uncoupling protein is functionally active, showing a nucleotide-sensitive conductance to protons and an uncoupling response to fatty acids. The amount of uncoupling protein in human mitochondria is equivalent to that in a partially cold-adapted guinea pig, indicating some potential for thermogenesis. Respiratory capacity measurements indicate that the total perinephric fat in adult man can only account for one-fivehundredth of the whole-body response to infused noradrenaline. Thus, although brown fat has been found to be quantitatively important in animal studies, considerable caution must be exercised in extrapolating its significance to adult man.