The oxygen consumption of testicular tissue of rabbit was measured under oral administration of three dose levels 6.5, 13 and 19.5 mg/kg body weight of thyroxine therapy. The oxygen consumption was correlated with the Sertoli cell ratio of the spermatogenic cell cycle as an index of exogenous function of the testis and the status of Leydig cells. 6.5 mg thyroxine resulted in lowered oxygen consumption and lowered Sertoli cell ratio of spermatogenic cell cycle. The other two doses 13 and 19.5 mg/kg body weight resulted in increased rate of oxygen consumption correlated with in creased Sertoli cell ratio of spermatogenic cycle and hyperplasia of Leydig cells. The increase in the oxygen consumption was proportional to the increase in Sertoli cell ratio. It was conclu ded that the increase in oxygen consumption under thyroxine therapy is related to the increase in number of testicular cells rather than to the increase metabolic enzymatic activity of oxidative processes.