The present study was carried out to investigate some histological changes of thymus gland of mature and senile rabbits. The study was carried out on fifteen apparently healthy mature (six-month-old) and senile (twelve-month-old) rabbits which were humanlysacrificed, eviscerated and subsequently tissue specimens from the thymuswere taken.It was found that the thymus of mature rabbits was lobulated organ consisted of stroma and parenchyma, the stroma consisted of connective tissue capsule and septa. The septal connective tissue rich in collagen fibers and contain low elastic fibers. Within the septal connective tissue found the muscular artery which characterized by duplicated internal elastic lamina. The artery branched to give small arteriole. Also, septa contained medium size vein. Parenchyma consisted of peripheral dark cortex and central light stained medulla. The cortex heavily populated by small lymphocytes which had dark nucleus and also contained epithelial reticular cells which had large vesicular nucleus. Medulla contained large lymphocytes, epithelial reticular cells and Hassall's corpuscles. The medullary epithelial reticular cells showed positive reaction for PAS stain. The thymus of senile rabbits showed signs ofinvolution, but it not completely disappeared. There were some changes, occurredthinning and irregularity of thymic lobule, progressive cortical thymocyte depletion, thymic cyst formation and lipid infiltrationwithin the capsule and septa. The present study revealed that thymus was present through all the life, after maturity occurred involution but complete absence of thymus not occurred.