Three doses of thyroxine 6.5, 13, 19.5 mg/kg body weight were given orally to three groups of Baladi rabbits, each group was 13, 11, 10 rabbits respectively. Five rabbits were used as control. The animals were slaughtered after 12 days. Testicular tissue sections were stained by P.A.S. The Sertoli cell ratio of the spermatogenic cell cycle, the number of leydig cells and the diameter of semineferous tubules were calculated. The first dose was statistically non significant. The seconed and third doses proportionally stimulated Sertoli cell, the division and differentiation of spermatogonia, sperma tocyte and spermatid. Physiological hyperplasia of Leydig cell was prominant.