2,4-Dinitrophenol, uniformly 14C-labeled, was injected intraperitoneally into rats 2 minutes before they were killed. Mitochondria prepared from their livers contained 14C. Almost all the 14C behaved like dinitrophenol-C 14 on two-dimensional thin-layer chromatography. Functional changes in the liver mitochondria (partial loss of respiratory control and of sensitivity to inhibition of respiration by added amytal) were consistent with the presence and amount of dinitrophenol. Hypothyroid rats, and normal and hypothyroid rats pretreated with l-thyroxine, were injected with dinitrophenol - 14C; the amount of 14C in their liver mitochondria per gram protein did not change from that in normal mitochondria. The increase in mitochondrial respiration as a function of the concentration of dinitrophenol added in successive increments was accomodated by the Hill equation. By the criterion of apparent stoichiometry in the Hill equation, the sensitivity of liver mitochondria to added dinitrophenol was controlled by the previous thyroid state of the rat: sensitivity was low in mitochondria from hypothyroid rats and was restored toward normal levels in mitochondria from hypothyroid rats killed 3 hours after injection with 5.2 mμg of l-thyroxine per gram or 2 minutes after injection with 5.2 μg of l-thyroxine per gram; sensitivity to dinitrophenol was raised above normal levels when normal rats were killed 6 hours after receiving 5.2 μg of l-thyroxine per gram. The respiratory responses of the liver mitochondria in vitro resembled the responses of the metabolic rate in vivo to dinitrophenol in previous reports.