Determination of blood testosterone level in three groups of male rabbits treated with three dose levels of thyroxine 6.5, 13 and 19.5 mg/kg body weight was performed. The thyroxine caused Leydig cell hyperplasia. The hyperplastic Leydig cells were evaluated to be biologically active by high Sertoli cell ratio of the spermato genic cell cycle. Although the testosterone level of the blood was decreased and uncorrelated with thyroxine injection, Leydig cells were proposed to secrete intratesticular testosterone.