Most of the resonances observed in heart mitochondria by e.p.r. spectroscopy at presently attainable sensitivities have been recently identified (for review, see [ 1 ] ) inculding the so-called iron-sulfur (Fe-S) ‘center 5’ [2], recognized by its low field resonance (gz = 2.086) in the reduced state of mitochondria from a variety of species and tissues [l] . The assignment of this e.p.r. line was based on the purification of a thus far unknown iron-sulfur (Fe-S) flavoprotein from beef heart mitochondria [3]. Since the purified Fe-S flavoprotein was most readily reduced by fatty acyl-CoA in the presence of acyl-CoA dehydrogenase and electron-transferring flavoprotein (ETF), it was suggested [3] that this Fe-S flavoprotein may function as an electron transfer protein between the fatty acyl-CoA dehydrogenase system [4] and the terminal part of the respiratory chain. In support of this conclusion, we would like to present further experimental evidence based on e.p.r. spectra of mitochondria from brown adipose tissue, a tissue highly specialized for fatty acid oxidation and non-shivering thermogenesis (for review, see [5]). Furthermore, we want to present results of e.p.r. measurements on whole brown adipose tissue during the entire period of adaptation to a cold environment, which establishes the suitability of e.p.r. spectroscopy as a non
