In the present investigation the relationship between developing ovary and thymus in Swiss mice was studied. To find out the relationship between ovary and thymus pregnant mice after confirming virginal plug were used. After parturition the ovary and thymus of their pups were removed on 1st, 21st, 35th and 49th day, fixed, embed and sections were prepared for the histological study. Thymus and ovary develops parallel until puberty (30th day) means cellular density of both organs is same. After puberty apparent cellular loss in the thymus of female mice is observed. While in case of males the inception of puberty takes place after 35th day of postnatal life. From the present investigation we conclude that ovary and thymus develops gradually up to puberty after that there is a retrogressive change in cellular numeral. The levels of gonadotropins are responsible for these changes; therefor onset of puberty in females occurs earlier in comparison to males. In the same species as the sexual maturity occurs on different postnatal days so the process of thymic involution is also takes place accordingly.