In this investigation, the morphological and ultra-structural variations occurred in thymuses of guinea pigs in postnatal period were examined. In the study, 10 groups, 1 day and 1, 2, 4, 6, 8, 12, 16, 20 and 24 weeks of groups consisting of 6 guinea pigs in each group (a total of 60 guinea pigs) were used.

One day old guinea pigs had a completely developed thymus. Cortex and medulla were easily distinguished. In cortex, a number of small lymphocytes, lymphoblasts, epithelial reticulum cells were seen whereas in medulla, epithelial reticulum cells and Hassall’s corpuscles were observed.

After birth, thymus weight increased parallel to body weight and reached to the maximum level in the sixth week. After the sixth week, a decrease in thymus weight was determined.

The lipid cells, one of the involution signs, were first seen in the sixth week. Together with the increasing age, increased amount of lipid tissue caused a decrease in cortex parenchyma as a result from entering the cortex and lipid infiltration. Hassall’s corpuscles decreased both in view of number and in view of volume. In medulla, a marked connective tissue was observed.

In conclusion, it was determined that thymus was completed its development at birth. It was also involuted after puberty and continued its functional activity during all life.