In the present investigation we would like to find out the histological relationship between testis and thymus during postnatal development in Swiss mice, which also, depends on hormonal physiology. To find out the relationship between testis and thymus we use pregnant mice after confirming virginal plug. After parturition the testis and thymus of their pups are removed on 1st, 21st, 35th and 49th day, fixed, embed and sections are prepared for the histological study. Thymus and testis develops parallel until puberty (35th day) means cellular density of both organs is same. After puberty apparent cellular loss in the thymus of male mice is observed. From the present investigation we conclude that testis and thymus develops gradually up to puberty after that there is a retrogressive change in cellular numeral. This type of histological and cellular interrelationship occurs due to the physiological activities of gonadal hormone.