BACKGROUND: The role of thyroid hormones has been recognized in normal embryo development many years ago. These hormones also affected the development of reproductive organs and their activity. Objectives: This study was conducted to investigate the effect of inducing hypothyroidism by propyl-2-thiouracyl (PTU) on lambs testicular histomorpho-logy and plasma testosterone concentration. Methods: Eighteen Lori-Bakhtari male lambs were divided to 3 groups (n=6) and each received one of the treatments as Control (C: 0 mg PTU/kg BW), Low (L: 10 mg PTU/kg BW) and High (H: 20 mg PTU/kg BW) by gavages, during a 60d experimental period. At the end of the trial, lambs were slaughtered and testes were removed to evaluate their histomorphological characteristics. Results: Mean concentration of T4 and T3 decreased significantly in L and H groups compared with C group (p<0.05). Hyphothyroidism increased testis weight, number of sertoli and lydig cells, diameter of Seminiferous tubules, diameter of seminiferous lumen, total spermatogonia, number of primary spermatocyte, and total spermatids (p<0.05). However, PTU had no significant effect on plasma concentration of testosterone (p>0.05). In the present study, PTU increased testicular weight and the number of cells involved in sperm and testosterone production. Conclusions: Sheep breeders may consider hypothyroidism as a potential approach to increase sperm production capacity in rams before puberty. However, further investigation should be carried out on the quality and quantity of rams' sperm.