An experiment was conducted to evaluate relative biopotency of tri-iodothyronine ($T_3$) and thyroxine ($T_4$) to induce oxygen consumption in young chicks. Four experimental groups of 3 chicks were injected with $T_3$ or $T_4$ at a dose of 500 or 1000mg per kg body weight, and thereafter oxygen consumption was measured by indirect calorimetry using a respiration apparatus. Oxygen consumption was significantly increased at 2 and 4 hour in the $T_3$ or $T_4$ treated chicks at 500mg at 2 hour. From coefficient of a multiple regression equation of oxygen consumption on $T_3$ or $T_4$, it was concluded that $T_3$ was shown to be two to three times as biologically active as $T_4$