Puberty is one of the most important stages in the postnatal development of mammals. There is paucity of age related histological studies in developing thymus. Due to the variation in the onset of pubertal development there is some basic difference in the development of thymus in male and female mice so present study deals with the developmental changes which occur in thymus of male Swiss mice. To evaluate the developing changes in the thymus healthy pregnant female Swiss mice were used. After parturition the thymus of their pups were removed on 1st, 21st, 35th and 49th day, fix, embed and sections prepare for the histological study. At the time of birth the components of thymus are very clear but no differentiation between the cortical and medullary components are apparent. On 35th day Cortical and medullary epithelial cells got their maximum number, largest size and appropriate shape. Different developing T-lymphocytes are present but immature and apoptotic cells are also found due to thymic involution on 49th day. We conclude that Right from birth to puberty there are progressive architectural changes in the thymic component but after puberty retrogressive changes noted due to age associated thymic atrophy. We also conclude that the gonadal hormones are responsible for this type of age related thymic involution.