Mitochondria were prepared from the brown adipose tissue of control rats and animals made iron deficient by means of a low iron diet. The specific activities of the mitochondrial electron transport system (NADH, succinate and a-glycerophosphate oxidase activities) were markedly and significantly reduced in preparations of brown adipose tissue from the iron-deficient rats as compared with preparations from the control animals. In contrast, concentrations of the cytochrome pigments a + a3, and c + c1were normal and cytochromebwas slightly reduced (18%) in the mitochondrial preparations from the iron-deficient animals. Treatment of the iron-deficient animals with triiodothyronine significantly increased the amount of brown fat present per kilogram of body weight in both control and iron-deficient rats, but did not significantly affect the specific activities of the mitochondrial electron transport system.